cmd/gc: use go.builtin as package prefix, not go%2ebuiltin

This matches all the other pseudo-packages.
The line was simply forgotten.

Change-Id: I278f6cbcfc883ea7efad07f99fc8c853b9b5d274
Reviewed-on: https://go-review.googlesource.com/4591Reviewed-by: Austin Clements <austin@google.com>

src/cmd/gc/lex.c

@@ -179,6 +179,7 @@ gcmain(int argc, char *argv[])
 	
 	// pseudo-package, for scoping
 	builtinpkg = mkpkg(newstrlit("go.builtin"));
+	builtinpkg->prefix = "go.builtin"; // not go%2ebuiltin
 
 	// pseudo-package, accessed by import "unsafe"
 	unsafepkg = mkpkg(newstrlit("unsafe"));