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/src/gb/gprs_ns2_internal.h b/src/gb/gprs_ns2_internal.h
index 08ffac2..e72deff 100644
--- a/src/gb/gprs_ns2_internal.h
+++ b/src/gb/gprs_ns2_internal.h
@@ -95,8 +95,12 @@
 
 	/*! workaround for rate counter until rate counter accepts char str as index */
 	uint32_t rate_ctr_idx;
+
+	/*! libmnl netlink socket for link state monitoring */
+	struct osmo_mnl *linkmon_mnl;
 };
 
+
 /*! Structure repesenting a NSE. The BSS/PCU will only have a single NSE, while SGSN has one for each BSS/PCU */
 struct gprs_ns2_nse {
 	uint16_t nsei;