helpers: fix printf type warning

There is no portable way to do this without a cast, and since
64bit filesystem magic numbers exist, cast to an at least 64bit
sized type (unsigned long long).

src/system/helpers.cpp

@@ -285,7 +285,7 @@ int check_dir(const char * dir, bool allow_tmp)
 				ret = 0;	// ok
 		}
 		if(ret == -1)
-			printf("Wrong Filessystem Type: 0x%" PRIx32"\n",s.f_type);
+			printf("Wrong Filessystem Type: 0x%llx\n", (unsigned long long)s.f_type);
 	}
 	return ret;
 }