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/.gitignore b/.gitignore
index 959b09b..234ef0b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -58,3 +58,5 @@
 debian/osmo-pcu.substvars
 debian/osmo-pcu/
 debian/tmp/
+
+osmo-pcu.pc