ANDROID: net: xfrm: always export pktgen_xfrm_outer_mode_output

Currently, it is not possible to set CONFIG_NET_PKTGEN=m in a
vendor kernel, as it depends on pktgen_xfrm_outer_mode_output,
which is only exported if CONFIG_NET_PKTGEN is enabled.

Always export this symbol so vendor kernels can depend on it.
This may be simpler than the alternative, which is to set
CONFIG_NET_PKTGEN=m in the GKI kernel.

Bug: 171693245
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Change-Id: I96b48c77121aea90fb4c4c51ecb97a92601bccf6