glsl: Delete builtin_builder::shader when destroying built-ins.

I would use _mesa_delete_shader, but it's declared static, and we don't
really need any of the stuff in it anyway.

This fixes a memory leak caught by Valgrind.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/glsl/builtin_functions.cpp

@@ -580,6 +580,9 @@ builtin_builder::release()
 {
    ralloc_free(mem_ctx);
    mem_ctx = NULL;
+
+   ralloc_free(shader);
+   shader = NULL;
 }
 
 void