rom.sh: don't run add_cbfs_option on dry builds

i added this in an earlier version of the patch, but
for some reason removed it.

this is necessary, or the build system will fail.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/rom.sh

@@ -236,7 +236,8 @@ mkcorebootbin_real()
 	fi
 
 	if [ -f "$cbfscfg" ]; then
-		dx_ add_cbfs_option "$cbfscfg"
+		$if_not_dry_build \
+			dx_ add_cbfs_option "$cbfscfg"
 	fi
 
 	if $if_not_dry_build grep "CONFIG_PAYLOAD_NONE=y" "$defconfig"; then