Add inter-thread queue

This adds an inter-thread queue "it_q" to libosmocore. With it_q,
one can perform thread-safe enqueing of messages to another thread,
who will receive the related messages triggered via an eventfd
handled in the usual libosmocore select loop abstraction.

Change-Id: Ie7d0c5fec715a2a577fae014b0b8a0e9c38418ef
diff --git a/src/Makefile.am b/src/Makefile.am
index 5ff1a42..dd31db8 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -28,6 +28,7 @@
 			 sockaddr_str.c \
 			 use_count.c \
 			 exec.c \
+			 it_q.c \
 			 $(NULL)
 
 if HAVE_SSSE3