x86: Fix warning in cmd_ximg.c when CONFIG_GZIP is not defined

This local variable is not used unless CONFIG_GZIP is defined. Fix it.
Signed-off-by: NSimon Glass <sjg@chromium.org>

common/cmd_ximg.c

@@ -58,7 +58,9 @@ do_imgextract(cmd_tbl_t * cmdtp, int flag, int argc, char * const argv[])
 	const void	*fit_data;
 	size_t		fit_len;
 #endif
+#ifdef CONFIG_GZIP
 	uint		unc_len = CONFIG_SYS_XIMG_LEN;
+#endif
 	uint8_t		comp;
 
 	verify = getenv_yesno("verify");