build: reference libcap from libzebra (BZ#393,626)

build: reference libcap from libzebra (BZ#393,626)

While the actual build failures have been fixed independently by
d1d3ac9 "build: reorder libraries to address linker error", libzebra
still does not reference libcap.  This will lead to more build failures
if someone else tries to use libzebra and doesn't add libcap.

Let's just add libcap here and be done with it.

I've not added libcap to the _DEPENDENCIES variable above since libcap
is a system library.  Actually, the whole _DEPENDENCIES thing is rather
fishy; automake automatically sets _DEPENDENCIES from _LIBADD.  For the
sake of not breaking stuff that works (especially since most autotools
stuff is arcane magic), I'm leaving it alone...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>