Added printf format checking to logging function.

This also required changing the logging of "" at the
start of each test to appeal the otherwise-friendly
compiler.

test-tool/test_prefetch16_simple.c

@@ -30,7 +30,7 @@ test_prefetch16_simple(void)
 {
 	int i, ret;
 
-	logging(LOG_VERBOSE, "");
+	logging(LOG_VERBOSE, LOG_BLANK_LINE);
 	logging(LOG_VERBOSE, "Test PREFETCH16 of 1-256 blocks at the start of the LUN");
 	for (i = 1; i <= 256; i++) {
 		ret = prefetch16(iscsic, tgt_lun, 0, i, 0, 0);