build: fix tentative generation of nft.8 after disabled doc

build: fix tentative generation of nft.8 after disabled doc

Despite doc generation is disabled, the makefile is trying to build it.

$ ./configure --disable-man-doc
$ make
Making all in doc
make[2]: Entering directory '/workdir/build-pkg/workdir/doc'
make[2]: *** No rule to make target 'nft.8', needed by 'all-am'.  Stop.
make[2]: Leaving directory '/workdir/build-pkg/workdir/doc'
make[1]: *** [Makefile:479: all-recursive] Error 1
make[1]: Leaving directory '/workdir/build-pkg/workdir'
make: *** [Makefile:388: all] Error 2

Fixes: 4f2813a313ae0 ("build: Include generated man pages in dist tarball")
Reported-by: Adan Marin Jacquot <adan.marin@zevenet.com>
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>