{xfrm,pktgen} Fix compiling error when CONFIG_XFRM is not set

0-DAY kernel build testing backend reported below error:
All error/warnings:

   net/core/pktgen.c: In function 'pktgen_if_write':
>> >> net/core/pktgen.c:1487:10: error: 'struct pktgen_dev' has no member named 'spi'
>> >> net/core/pktgen.c:1488:43: error: 'struct pktgen_dev' has no member named 'spi'

Fix this by encapuslating the code with CONFIG_XFRM.

Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/core/pktgen.c

@@ -1482,7 +1482,7 @@ static ssize_t pktgen_if_write(struct file *file,
 		sprintf(pg_result, "OK: flows=%u", pkt_dev->cflows);
 		return count;
 	}
-
+#ifdef CONFIG_XFRM
 	if (!strcmp(name, "spi")) {
 		len = num_arg(&user_buffer[i], 10, &value);
 		if (len < 0)
@@ -1493,7 +1493,7 @@ static ssize_t pktgen_if_write(struct file *file,
 		sprintf(pg_result, "OK: spi=%u", pkt_dev->spi);
 		return count;
 	}
-
+#endif
 	if (!strcmp(name, "flowlen")) {
 		len = num_arg(&user_buffer[i], 10, &value);
 		if (len < 0)