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: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_ximg.c b/common/cmd_ximg.c
index ea0a26e..02084b0 100644
--- a/common/cmd_ximg.c
+++ b/common/cmd_ximg.c
@@ -58,7 +58,9 @@
 	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");