utils_cmd_flush.c: fix build warning

utils_cmd_flush.c: fix build warning

CC       src/utils_cmd_flush.lo
src/utils_cmd_flush.c:139:12: warning: declaration shadows a local variable [-Wshadow]
      char buffer[1024];
           ^
src/utils_cmd_flush.c:108:47: note: previous declaration is here
cmd_status_t cmd_handle_flush(FILE *fh, char *buffer) {
                                              ^