Add pkg-config file

We're installing header file pcuif_proto.h so it's better to use
pkg-config for proper version tracking similar to the way it's done for
OpenBSC.

Change-Id: I0520045e5655794df152b98b9755d7cbbd334049
diff --git a/debian/osmo-pcu.install b/debian/osmo-pcu.install
index 768719c..54dcc63 100644
--- a/debian/osmo-pcu.install
+++ b/debian/osmo-pcu.install
@@ -1,3 +1,4 @@
 etc/osmocom/osmo-pcu.cfg
 usr/bin/osmo-pcu
 usr/include/osmocom/pcu/pcuif_proto.h
+usr/lib/pkgconfig/osmo-pcu.pc