vm_page.c: include opt_msgbuf.h for MSGBUF_SIZE use in vm_page_startup

vm_page_startup uses MSGBUF_SIZE value for adding msgbuf pages to minidump.
If opt_msgbuf.h is not included and MSGBUF_SIZE is overriden in kernel
config, then not all msgbuf pages will be dumped.  And most importantly,
struct msgbuf itself will not be included.  Thus the dump would look
corrupted/incomplete to tools like kgdb, dmesg, etc that try to access
struct msgbuf as one of the first things they do when working on a crash
dump.

MFC after:	5 days

sys/vm/vm_page.c

@@ -100,6 +100,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include "opt_msgbuf.h"
 #include "opt_vm.h"
 
 #include <sys/param.h>