build: Make action targets hermetic

build: Make action targets hermetic

Fuchsia GN build requires all "action" targets to be
hermetic, and they should correctly and fully state
their inputs and ouptuts in "sources" and "outputs"
fields (see https://fuchsia.dev/fuchsia-src/development/build/hermetic_actions
for details).

This change adds "sources" field to "glslang_extension_headers"
build target so that it states all its input files in its
GN target build rule.

TEST=fx set workstation.x64 --args=build_should_trace_actions=true
     fx build

Bug: 90846
Change-Id: I63bd5d03cee86d93b0bb7cf2cee95c5ae0d98f93