cbfstool: Fix compile issue for older gcc versions

gcc 4.4.7 fails to compile due to the missing initializers
for all struct members. Add initializers for all fields.

Change-Id: If1ad4fff0f965ccd7e821820c0703853c1e5c590
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/13418
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

util/cbfstool/fmaptool.c

@@ -126,7 +126,7 @@ int main(int argc, char **argv)
 		// Optional
 		const char *header_filename;
 		const char *region_filename;
-	} args = {NULL};
+	} args = {NULL, NULL, NULL, NULL};
 
 	bool show_usage = false;
 	int each_arg;