Add tests for pcu_lsb()

This utility functions is used by TBF allocation routines and only
tested indirectly through allocation test. Let's add proper exhaustive
test which checks all uint8_t values.

This also requires adding missing include to pcu_utils.h

Change-Id: If08a7f0d31f0e5ad8a5efa5885880aed19c329ab
diff --git a/src/pcu_utils.h b/src/pcu_utils.h
index 3a64a47..767771c 100644
--- a/src/pcu_utils.h
+++ b/src/pcu_utils.h
@@ -16,6 +16,10 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  */
 
+extern "C" {
+#include <osmocom/gsm/gsm_utils.h>
+}
+
 inline int msecs_to_frames(int msecs) {
 	return (msecs * (1024 * 1000 / 4615)) / 1024;
 }