Fix kernel build on amd64 after r256118, since the machine/md_var.h

header is not implicitly included there.  So include it explicitly.

Approved by:	re (delphij)
Pointy hat to:	dim
MFC after:	3 days
X-MFC-With:	r256118

sys/dev/cxgbe/t4_sge.c

@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <netinet/ip.h>
 #include <netinet/ip6.h>
 #include <netinet/tcp.h>
+#include <machine/md_var.h>
 
 #include "common/common.h"
 #include "common/t4_regs.h"