Fix nooptions VIMAGE build

The recent FIBify commits deref struct thread without including
sys/proc.h, which can result in a compiler error.  This becomes
apparent when building with LINT-NOVIMAGE, as net/vnet.h includes
sys/proc.h.  Fix this by directly including sys/proc.h

Fixes: 4c486fe402 ("ip_mroute: FIBify"), 0bb9c2b665 ("ip6_mroute: FIBify")

sys/netinet/ip_mroute.c

@@ -85,6 +85,7 @@
 #include <sys/mbuf.h>
 #include <sys/module.h>
 #include <sys/priv.h>
+#include <sys/proc.h>
 #include <sys/protosw.h>
 #include <sys/signalvar.h>
 #include <sys/socket.h>

sys/netinet6/ip6_mroute.c

@@ -91,6 +91,7 @@
 #include <sys/module.h>
 #include <sys/domain.h>
 #include <sys/priv.h>
+#include <sys/proc.h>
 #include <sys/protosw.h>
 #include <sys/sdt.h>
 #include <sys/signalvar.h>