Fix compiler warning in check_update.c

Fixes #263
The conditional checking for network connection always evaluated as true
since the check_network() function was not being called. Add the
parentheses to actually check for network.

Signed-off-by: Matthew Johnson <matthew.johnson@intel.com>

src/check_update.c

@@ -139,7 +139,7 @@ static int check_update()
 	}
 	swupd_curl_init();
 
-	if (!check_network) {
+	if (!check_network()) {
 		fprintf(stderr, "Error: Network issue, unable to proceed with update\n");
 		return ENOSWUPDSERVER;
 	}