GN: Make glslang_validator an executable.

The target was accidentally listed as a source_set.
diff --git a/BUILD.gn b/BUILD.gn
index 3e061ac..26ab5c3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -169,7 +169,7 @@
   public_configs = [ ":glslang_public" ]
 }
 
-source_set("glslang_validator") {
+executable("glslang_validator") {
   sources = [
     "StandAlone/DirStackFileIncluder.h",
     "StandAlone/StandAlone.cpp",