vp10_convolve_ssse3.c: make some functions static
vp10_convolve_ssse3.c: make some functions static

quiets -Wmissing-prototypes warnings

BUG=b/29584271

Change-Id: I4d2eb7f4b45d7b829421976641b3212bcf29e7dd

GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom