igb(4): Fix define and includes with RSS option enabled

This re-adds the opt_rss.h header to the driver and includes some
RSS-specific headers when RSS is defined.

PR:		249191
Submitted by:	Milosz Kaniewski <milosz.kaniewski@gmail.com>
MFC after:	3 days

sys/dev/e1000/if_em.h

@@ -30,6 +30,7 @@
 #include "opt_ddb.h"
 #include "opt_inet.h"
 #include "opt_inet6.h"
+#include "opt_rss.h"
 
 #ifdef HAVE_KERNEL_OPTION_HEADERS
 #include "opt_device_polling.h"
@@ -69,6 +70,10 @@
 #include <net/if_dl.h>
 #include <net/if_media.h>
 #include <net/iflib.h>
+#ifdef	RSS
+#include <net/rss_config.h>
+#include <netinet/in_rss.h>
+#endif
 
 #include <net/if_types.h>
 #include <net/if_vlan_var.h>