renderer: silence unused variable warning

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Elie Tournier <elie.tournier@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/vrend_renderer.c

@@ -7953,7 +7953,7 @@ static void vrender_get_glsl_version(int *glsl_version)
 {
    int major_local, minor_local;
    const GLubyte *version_str;
-   int c;
+   MAYBE_UNUSED int c;
    int version;
 
    version_str = glGetString(GL_SHADING_LANGUAGE_VERSION);