dln.c: xmalloc_mismatch_p with dlopen

* dln.c (xmalloc_mismatch_p): define only when using dlopen,
  otherwise dlsym is not available too, and should be used then.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61576 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

dln.c

@@ -1243,7 +1243,8 @@ rb_w32_check_imported(HMODULE ext, HMODULE mine)
 #define translit_separator(str) (void)(str)
 #endif
 
-MAYBE_UNUSED(static bool xmalloc_mismatch_p(void *handle));
+#ifdef USE_DLN_DLOPEN
+static bool xmalloc_mismatch_p(void *handle);
 
 #ifdef __clang__
 #pragma clang diagnostic push
@@ -1263,6 +1264,7 @@ xmalloc_mismatch_p(void *handle)
 #elif defined(__GNUC__) && (__GNUC__ >= 5)
 #pragma GCC diagnostic pop
 #endif
+#endif
 
 void*
 dln_load(const char *file)