Add a missing FORTIFY_PEDANTIC_CHECKSexperimental

Fixing the following issue:

```
In file included from xstrndup.c:34:
/OpenWrt/aarch64/staging_dir/toolchain-aarch64_cortex-a53_gcc-14.3.0_musl/include/fortify/string.h: In function 'mempcpy':
/OpenWrt/aarch64/staging_dir/toolchain-aarch64_cortex-a53_gcc-14.3.0_musl/include/fortify/string.h:340:13: error: 'nonnull' argument '__d' compared to NULL [-Werror=nonnull-compare]
  340 |         if (!__d || !__s)
      |             ^~~~
/OpenWrt/aarch64/staging_dir/toolchain-aarch64_cortex-a53_gcc-14.3.0_musl/include/fortify/string.h:340:21: error: 'nonnull' argument '__s' compared to NULL [-Werror=nonnull-compare]
  340 |         if (!__d || !__s)
      |                     ^~~~
cc1: all warnings being treated as errors
make[5]: *** [Makefile:511: xstrndup.o] Error 1
```

Reported in https://github.com/openwrt/openwrt/pull/20552

1 files changed, 2 insertions, 0 deletions

diff --git a/include/string.h b/include/string.h
index 7a8564a..98ec292 100644
--- a/include/string.h
+++ b/include/string.h
@@ -344,8 +344,10 @@ _FORTIFY_FN(mempcpy) void *mempcpy(void * _FORTIFY_POS0 __d,
 #if __has_builtin(__builtin___mempcpy_chk) && defined(FORTIFY_USE_NATIVE_CHK)
         return __builtin___mempcpy_chk(__d, __s, __n, __fh_bos(__d, 0));
 #else
+#if defined FORTIFY_PEDANTIC_CHECKS
         if (!__d || !__s)
                 __builtin_trap();
+#endif
 
         __fh_size_t __bd = __fh_bos(__d, 0);
         __fh_size_t __bs = __fh_bos(__s, 0);