undefined symbol persist_mode_main when HAVE_NETLINK=0

[andreasbaumann]

Reproducable when manually setting HAVE_NETLINK to 0 in config.h:

nbd-client.c: In function ‘main’:
nbd-client.c:1549:32: error: implicit declaration of function ‘persist_mode_main’ [-Wimplicit-function-declaration]
 1549 |                         return persist_mode_main(index, sockfds, flags);
      |                                ^~~~~~~~~~~~~~~~~

[yoe]

In response to this mistake, I've made netlink not optional in commit 6a48271. This should have happened years ago.

You must make sure that libnl-genl is installed and that its .pc file is available in the pkg-config (or pkgconf) search path for this to work.