Include all SkSL sources when building Skia.

This is the equivalent of https://crrev.com/669823

TBR=reed@google.com
Change-Id: I1724599b0203c67f2a2b7f6b827ea2661a1f8d59
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/57735
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

skia/BUILD.gn

diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index ca1bac1..6b315d9 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -160,6 +160,7 @@
   ]
 
   # The skia sources values are relative to the skia_dir, so we need to rebase.
+  sources += skia_sksl_gpu_sources
   sources += skia_sksl_sources
   sources += skia_utils_sources
   sources += [