mesa: fix unused variable warning in release builds

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/mesa/main/uniforms.c

@@ -104,7 +104,8 @@ _mesa_update_shader_textures_used(struct gl_shader_program *shProg,
    GLbitfield mask = prog->SamplersUsed;
    gl_shader_stage prog_stage =
       _mesa_program_enum_to_shader_stage(prog->Target);
-   struct gl_linked_shader *shader = shProg->_LinkedShaders[prog_stage];
+   MAYBE_UNUSED struct gl_linked_shader *shader =
+      shProg->_LinkedShaders[prog_stage];
    GLuint s;
 
    assert(shader);