fdgrowtable_test.c: appease gcc

Work around bogus gcc -Wreturn-type.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44511

Reviewed by:	kevans, rew
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D27563

tests/sys/kern/fdgrowtable_test.c

@@ -151,7 +151,7 @@ ATF_TC_BODY(free_oldtables, tc)
 	ATF_CHECK(old_tables(kd,kp) == 0);
 }
 
-static void *
+static _Noreturn void *
 exec_thread(void *args)
 {
 	for (;;)