examples: compile with `make check' and add AM_CPPFLAGS

Compile examples via `make check' like libnftnl does. Use AM_CPPFLAGS to
specify local headers via -I.

Unfortunately, `make distcheck' did not catch this compile time error in
my system, since it was using the nftables/libnftables.h file of the
previous nftables release.

Fixes: 5b364657a35f ("build: missing SUBIRS update")
Fixes: caf2a6ad2d22 ("examples: add libnftables example program")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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