ANDROID: Fix build-break caused by android_rvh_nf_conn_alloc hook

The new android_rvh_nf_conn_alloc hook was not declared properly
so kernelci builds break if !CONFIG_ANDROID_VENDOR_HOOKS.

Fixes: 01435b2e ("ANDROID: GKI: net: add vendor hooks
for 'struct nf_conn' lifecycle")
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I1208dc440836687a9c5621323ff1cf5a5bec862b