netfilter.mk: add version conditional for CONFIG_NF_CT_PROTO_GRE

netfilter.mk: add version conditional for CONFIG_NF_CT_PROTO_GRE

Kernel commit 22fc4c4c9fd6 ("netfilter: conntrack: gre: switch module to
be built-in") moved the CT GRE code into the core nf_conntrack.ko module
and changed the CONFIG_NF_CT_PROTO_GRE option to boolean for kernel 5.1
and onwards.

CONFIG_NF_CT_PROTO_GRE at the moment has no prompt and can only be
selected by NF_CONNTRACK_PPTP

Fixes: FS#2990 (partial)
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2990

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
[note that the option now can not be enabled on its own]
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>