gallium/util: add missing include

This source-file uses PIPE_OS_WINDOWS to enable the Windows
functionality. But witout including p_config.h, this pre-processor
symbol won't be defined at all.

Let's fix this by adding the missing include, enabling stack-traces on
Windows.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5497>

src/gallium/auxiliary/util/u_debug_stack.c

@@ -35,6 +35,7 @@
 #include "util/u_debug.h"
 #include "u_debug_symbol.h"
 #include "u_debug_stack.h"
+#include "pipe/p_config.h"
 
 #if defined(HAVE_LIBUNWIND)