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/include/Makefile.am b/include/Makefile.am
index 842b872..c1ae644 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -30,6 +30,7 @@
                        osmocom/core/hash.h \
                        osmocom/core/hashtable.h \
                        osmocom/core/isdnhdlc.h \
+                       osmocom/core/it_q.h \
                        osmocom/core/linuxlist.h \
                        osmocom/core/linuxrbtree.h \
                        osmocom/core/log2.h \