Fix build with C23 / GCC 15+: ATOMIC_VAR_INIT was removed in C23.

Upstream munit is unmaintained since 2020, so patch in-tree.

--- a/test/munit/munit.c
+++ b/test/munit/munit.c
@@ -798,7 +798,7 @@
 #elif defined(HAVE_STDATOMIC)
 #  include <stdatomic.h>
 #  define ATOMIC_UINT32_T _Atomic uint32_t
-#  define ATOMIC_UINT32_INIT(x) ATOMIC_VAR_INIT(x)
+#  define ATOMIC_UINT32_INIT(x) (x)
 #elif defined(HAVE_CLANG_ATOMICS)
 #  define ATOMIC_UINT32_T _Atomic uint32_t
 #  define ATOMIC_UINT32_INIT(x) (x)