Forward declare 'struct cpl_set_tcb_rpl' before including t4_tom.h.

Other structures needed by prototypes in t4_tom.h are explicitly
declared in this file, so adding the prototype here seems most
consistent with existing code.

sys/dev/cxgbe/iw_cxgbe/cm.c

@@ -57,6 +57,7 @@ __FBSDID("$FreeBSD$");
 
 struct sge_iq;
 struct rss_header;
+struct cpl_set_tcb_rpl;
 #include <linux/types.h>
 #include "offload.h"
 #include "tom/t4_tom.h"