include: drop c11_compat.h

We now require C11, and C++ supports static_assert just fine, which is
the only thing this header ever added support for. So let's get rid of
this needless header.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16812>

src/util/macros.h

@@ -26,9 +26,6 @@
 
 #include <stdio.h>
 #include <assert.h>
-
-#include "c11_compat.h"
-
 #include <stdint.h>
 
 /* Compute the size of an array */