Fix link error when running `make tools` with skia_shared_lib=1
Fix link error when running `make tools` with skia_shared_lib=1

I'm getting this error, which is fixed by this change:

[75/76] LINK skpdiff
FAILED: c++ -m64 -pie -Wl,-rpath=\$ORIGIN/lib/ -Wl,-rpath-link=lib/ -o skpdiff -Wl,--start-group obj/tools/skpdiff/skpdiff.skpdiff_main.o obj/tools/skpdiff/skpdiff.SkDiffContext.o obj/tools/skpdiff/skpdiff.SkImageDiffer.o obj/tools/skpdiff/skpdiff.SkPMetric.o obj/tools/skpdiff/skpdiff.skpdiff_util.o obj/tools/skpdiff/skpdiff.SkDifferentPixelsMetric_cpu.o obj/gyp/libflags.a obj/gyp/libpicture_utils.a -Wl,--end-group lib/libskia.so -lrt -lz
/usr/bin/ld: obj/tools/skpdiff/skpdiff.SkDiffContext.o: undefined reference to symbol 'pthread_mutexattr_settype@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

BUG=skia:

Review URL: https://codereview.chromium.org/1206333002

rubenvb/skia,google/skia,noselhq/skia,rubenvb/skia,vanish87/skia,HalCanary/skia-hc,ominux/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,shahrzadmn/skia,shahrzadmn/skia,ominux/skia,nvoron23/skia,todotodoo/skia,vanish87/skia,ominux/skia,pcwalton/skia,google/skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,qrealka/skia-hc,noselhq/skia,Hikari-no-Tenshi/android_external_skia,google/skia,todotodoo/skia,tmpvar/skia.cc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,vanish87/skia,shahrzadmn/skia,shahrzadmn/skia,rubenvb/skia,nvoron23/skia,Jichao/skia,shahrzadmn/skia,qrealka/skia-hc,google/skia,noselhq/skia,pcwalton/skia,noselhq/skia,google/skia,noselhq/skia,aosp-mirror/platform_external_skia,shahrzadmn/skia,noselhq/skia,Jichao/skia,pcwalton/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,HalCanary/skia-hc,nvoron23/skia,tmpvar/skia.cc,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,ominux/skia,ominux/skia,todotodoo/skia,aosp-mirror/platform_external_skia,Jichao/skia,ominux/skia,noselhq/skia,HalCanary/skia-hc,vanish87/skia,qrealka/skia-hc,Jichao/skia,Jichao/skia,shahrzadmn/skia,vanish87/skia,nvoron23/skia,pcwalton/skia,HalCanary/skia-hc,tmpvar/skia.cc,todotodoo/skia,shahrzadmn/skia,noselhq/skia,shahrzadmn/skia,qrealka/skia-hc,todotodoo/skia,Hikari-no-Tenshi/android_external_skia,todotodoo/skia,Jichao/skia,todotodoo/skia,rubenvb/skia,Jichao/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,vanish87/skia,vanish87/skia,HalCanary/skia-hc,todotodoo/skia,pcwalton/skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,ominux/skia,pcwalton/skia,qrealka/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Jichao/skia,vanish87/skia,nvoron23/skia,todotodoo/skia,pcwalton/skia,Jichao/skia,aosp-mirror/platform_external_skia,ominux/skia,nvoron23/skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,pcwalton/skia,qrealka/skia-hc,HalCanary/skia-hc,ominux/skia,vanish87/skia,pcwalton/skia,nvoron23/skia,google/skia,nvoron23/skia,qrealka/skia-hc,noselhq/skia,rubenvb/skia,tmpvar/skia.cc,tmpvar/skia.cc,qrealka/skia-hc,rubenvb/skia