Include all SkSL sources when building Skia

SkSL is being split into CPU and GPU sub-components, this is necessary
to continue building the entire thing.

Change-Id: Id58e4d0a7a68504a76be28f27df605492602f47b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661369
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/master@{#669823}

skia/BUILD.gn

@@ -523,6 +523,7 @@ component("skia") {
     sources += skia_gpu_sources
     sources += skia_null_gpu_sources
     sources += skia_sksl_sources
+    sources += skia_sksl_gpu_sources
     if (enable_vulkan) {
       sources += skia_vk_sources
       sources += [