util: fix error about missing include

When building with linux_glibc_x86_64 in AOSP, this is
observed.

src/util/u_debug_stack.h:66:4: error: unknown type name 'uint64_t'
   66 |    uint64_t start_ip;
      |    ^

Adding <stdint.h> fixes it.

Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38644>

src/util/u_debug_stack.h

@@ -29,6 +29,7 @@
 #define U_DEBUG_STACK_H_
 
 #include <stdio.h>
+#include <stdint.h>
 
 #include "util/detect_os.h"