lib: consolidate flexible array hack in a single place

lib: consolidate flexible array hack in a single place

Old gcc versions (< 5.x) have a bug that prevents C99 flexible
arrays from working properly on shared libraries.

We already have a hack in place to work around this problem, but it
needs to be replicated in every declaration of a frr_yang_module_info
variable within libfrr. This clearly isn't a good solution if we
consider that many more libfrr YANG modules are about to come in
the future.

This commit introduces a different workaround that operates within
the northbound layer itself, such that implementers of libfrr YANG
modules won't need to worry about this problem anymore.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>