kern_exec.c: explicitly include sys/limits.h for UINT_MAX

While there, remove unneeded manual inclusion of sys/cdefs.h.

Fixes:	e1a84b7708 ("execve_block(): a mechanism for mutual exclusion with execve() on the process")
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/kern_exec.c

@@ -26,7 +26,6 @@
  * SUCH DAMAGE.
  */
 
-#include <sys/cdefs.h>
 #include "opt_capsicum.h"
 #include "opt_hwpmc_hooks.h"
 #include "opt_hwt_hooks.h"
@@ -46,6 +45,7 @@
 #include <sys/imgact.h>
 #include <sys/imgact_elf.h>
 #include <sys/kernel.h>
+#include <sys/limits.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
 #include <sys/mman.h>