Fix GCC 7 warning, convert "Fall through" to "fallthrough" comment

In GCC 7 the -Wextra infers the new -Wimplicit-fallthrough which can be
suppressed by adding a simple comment

    /* fallthrough */

Which we had, but was too verbose (and misspelled) for GCC to notice.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/initctl.c

@@ -131,8 +131,8 @@ static void parse(void *arg, int fd, int events)
 			switch (rq.runlevel) {
 			case 's':
 			case 'S':
-				rq.runlevel = '1';
-				/* Fall through to regular processing */
+				rq.runlevel = '1'; /* Single user mode */
+				/* fallthrough */
 
 			case '0'...'9':
 				_d("Setting new runlevel %c", rq.runlevel);