{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>