selftests: watchdog: fix mixed whitespace

Convert spaces to tabs for checkpatch compliance. Quick way to verify
this is by running `git show -w <commit-id>`, which returns an empty
commit body. No functional change intended.
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/watchdog/watchdog-test.c

@@ -99,7 +99,7 @@ int main(int argc, char *argv[])
 
 	signal(SIGINT, term);
 
-    while(1) {
+	while (1) {
 		keep_alive();
 		sleep(ping_rate);
 	}