Integrate libmnl (minimal netlink) library with libosmocore select loop

This adds an easy way to listen to netlink events form the Linux kernel
from within libosmocore applications.

The new dependency can be disabled via the "--disable-lbimnl" configure flag.

Change-Id: I4f787ee68f0d6d04f0a5655eb57d55b3b326a42f
diff --git a/contrib/libosmocore.spec.in b/contrib/libosmocore.spec.in
index fb45516..728a0fb 100644
--- a/contrib/libosmocore.spec.in
+++ b/contrib/libosmocore.spec.in
@@ -30,6 +30,7 @@
 BuildRequires:  pkgconfig(libpcsclite)
 BuildRequires:  pkgconfig(libusb-1.0)
 BuildRequires:  pkgconfig(talloc) >= 2.0.1
+BuildRequires:  pkgconfig(libmnl)
 
 %description
 libosmocore is a package with various utility functions that were