main: fix a regression in uniform handling introduced by 87a4bc5

The comment was accurate but the condition was reversed...

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>

src/mesa/program/ir_to_mesa.cpp

@@ -2410,7 +2410,7 @@ _mesa_associate_uniform_storage(struct gl_context *ctx,
          &shader_program->UniformStorage[location];
 
       /* Do not associate any uniform storage to built-in uniforms */
-      if (!storage->builtin)
+      if (storage->builtin)
          continue;
 
       if (location != last_location) {