libnftables: Make some functions globally accessible

libnftables: Make some functions globally accessible

This removes static flag and adds header prototype for the following
functions:

* must_print_eq_op() from src/expression.c
* fib_result_str() from src/fib.c
* set_policy2str() and chain_policy2str from src/rule.c

In fib.h, include linux/netfilter/nf_tables.h to make sure enum
nft_fib_result is known when including this file.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>