Use updated gni source list (flutter/engine#56627)

In https://skia-review.googlesource.com/c/skia/+/884096, Skia broke up
this gni file list into Ganesh and Graphite specific code. Since Flutter
only needs Ganesh sources, this can be made more precise.

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engine/src/flutter/skia/BUILD.gn

@@ -649,7 +649,7 @@ skia_component("skia") {
   sources += skia_codec_decode_bmp
   sources += skia_encode_srcs
   sources += skia_sksl_core_sources
-  sources += skia_sksl_default_module_sources
+  sources += skia_sksl_core_module_sources
   sources += skia_ports_sources
   sources += [
     "$_skia_root/src/android/SkAndroidFrameworkUtils.cpp",