Perl_my_vsnprintf: avoid compiler warning

in the usequadmath branch, gcc is too clever for its own good:

    PERL_UNUSED_ARG(ap);

gives:

util.c:5299:18: warning: ‘sizeof’ on array function parameter ‘ap’ will
return size of ‘__va_list_tag *’ [-Wsizeof-array-argument]

Stick in a void* cast to shut it up.

util.c

@@ -5296,7 +5296,8 @@ Perl_my_vsnprintf(char *buffer, const Size_t len, const char *format, va_list ap
     PERL_UNUSED_ARG(buffer);
     PERL_UNUSED_ARG(len);
     PERL_UNUSED_ARG(format);
-    PERL_UNUSED_ARG(ap);
+    /* the cast is to avoid gcc -Wsizeof-array-argument complaining */
+    PERL_UNUSED_ARG((void*)ap);
     Perl_croak_nocontext("panic: my_vsnprintf not available with quadmath");
     return 0;
 #else