added skslc to BUILD.gn
added skslc to BUILD.gn

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4764

Change-Id: I89330cfa4be4d43d9183a8e53b28d6c0061f33e2
Reviewed-on: https://skia-review.googlesource.com/4764
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>

aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,google/skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia