Fix build failure with clang < 17:

./crau/crau.h:263:60: error: expected ')'
void crau_push_context(struct crau_context_stack_st *stack CRAU_MAYBE_UNUSED,
                                                           ^
./crau/crau.h:263:23: note: to match this '('
void crau_push_context(struct crau_context_stack_st *stack CRAU_MAYBE_UNUSED,
                      ^

https://gitlab.com/gnutls/gnutls/-/work_items/1880
--- lib/crau/crau.h.orig	2026-04-29 09:33:25.000000000 -0500
+++ lib/crau/crau.h	2026-04-30 21:06:11.000000000 -0500
@@ -255,10 +255,16 @@
 #    if __has_c_attribute (__maybe_unused__)
 #     define CRAU_MAYBE_UNUSED [[__maybe_unused__]]
 #    endif
-#   elif defined(__GNUC__)
+#   endif
+#  endif /* CRAU_MAYBE_UNUSED */
+#  ifndef CRAU_MAYBE_UNUSED
+#   ifdef __GNUC__
 #    define CRAU_MAYBE_UNUSED __attribute__((__unused__))
 #   endif
 #  endif /* CRAU_MAYBE_UNUSED */
+#  ifndef CRAU_MAYBE_UNUSED
+#   define CRAU_MAYBE_UNUSED
+#  endif /* CRAU_MAYBE_UNUSED */
 
 void crau_push_context(struct crau_context_stack_st *stack CRAU_MAYBE_UNUSED,
 		       long context CRAU_MAYBE_UNUSED)
