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/Makefile.am b/Makefile.am
index 12cb478..1536016 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,3 +3,5 @@
 SUBDIRS = include src examples tests
 EXTRA_DIST = osmoappdesc.py
 
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = osmo-pcu.pc