netfilter: bridge: define INT_MIN & INT_MAX in userspace

With 4.19, programs like ebtables fail to build when they include
"linux/netfilter_bridge.h". It is caused by commit 94276fa8 which
added a use of INT_MIN and INT_MAX to the header:
: In file included from /usr/include/linux/netfilter_bridge/ebtables.h:18,
:                  from include/ebtables_u.h:28,
:                  from communication.c:23:
: /usr/include/linux/netfilter_bridge.h:30:20: error: 'INT_MIN' undeclared here (not in a function)
:   NF_BR_PRI_FIRST = INT_MIN,
:                     ^~~~~~~

Define these constants by including "limits.h" when !__KERNEL__ (the
same way as for other netfilter_* headers).

Fixes: 94276fa8 ("netfilter: bridge: Expose nf_tables bridge hook priorities through uapi")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>