Require uint32_t alignment for ipfw_insn

There are many casts of this struct to uint32_t, so we also need to ensure
that it is sufficiently aligned to safely perform this cast on architectures
that don't allow unaligned accesses. This fixes lots of -Wcast-align warnings.

Reviewed By:	ae
Differential Revision: https://reviews.freebsd.org/D27879

sys/netinet/ip_fw.h

@@ -339,7 +339,7 @@ enum ipfw_opcodes {		/* arguments (4 byte each)	*/
  *
  */
 typedef struct	_ipfw_insn {	/* template for instructions */
-	u_int8_t 	opcode;
+	_Alignas(_Alignof(u_int32_t)) u_int8_t 	opcode;
 	u_int8_t	len;	/* number of 32-bit words */
 #define	F_NOT		0x80
 #define	F_OR		0x40