[linux] Fix building with kernel 4.11 headers

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/linux/af_packet.c

 #include <ipxe/socket.h>
 
 /* This hack prevents pre-2.6.32 headers from redefining struct sockaddr */
+#define _SYS_SOCKET_H
 #define __GLIBC__ 2
 #include <linux/socket.h>
 #include <linux/if.h>

src/drivers/linux/tap.c

 #include <ipxe/socket.h>
 
 /* This hack prevents pre-2.6.32 headers from redefining struct sockaddr */
+#define _SYS_SOCKET_H
 #define __GLIBC__ 2
 #include <linux/socket.h>
 #undef __GLIBC__