add libosmo-mslookup abstract client

mslookup is a key concept in Distributed GSM, which allows querying the current
location of a subscriber in a number of cooperating but independent core
network sites, by arbitrary service names and by MSISDN/IMSI.

Add the abstract mslookup client library. An actual lookup method (besides
mslookup_client_fake.c) is added in a subsequent patch.

For a detailed overview of this and upcoming patches, please see the elaborate
comment at the top of mslookup.c.

Add as separate library, libosmo-mslookup, to allow adding D-GSM capability to
arbitrary client programs.

osmo-hlr will be the only mslookup server implementation, added in a subsequent
patch.

osmo-hlr itself will also use this library and act as an mslookup client, when
requesting the home HLR for locally unknown IMSIs.

Related: OS#4237
Patch-by: osmith, nhofmeyr
Change-Id: I83487ab8aad1611eb02e997dafbcb8344da13df1
diff --git a/include/Makefile.am b/include/Makefile.am
index d8eb1ec..e9a7126 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -1,3 +1,8 @@
 SUBDIRS = osmocom
 
-nobase_include_HEADERS = osmocom/gsupclient/gsup_client.h
+nobase_include_HEADERS = \
+	osmocom/gsupclient/gsup_client.h \
+	osmocom/mslookup/mslookup_client_fake.h \
+	osmocom/mslookup/mslookup_client.h \
+	osmocom/mslookup/mslookup.h \
+	$(NULL)