Install the pcuif_proto.h header file

So far, we used to keep a copy of the header file around in
both osmo-pcu and osmo-bts projects.  Before we start introducing
a third copy in openbsc, let's have the osmo-pcu install the header
file and make the other programs use that.

Change-Id: I60976c9be5488256d1ff55fdc5aa548e3705400d
diff --git a/src/osmobts_sock.cpp b/src/osmobts_sock.cpp
index 21a404f..f589dc2 100644
--- a/src/osmobts_sock.cpp
+++ b/src/osmobts_sock.cpp
@@ -35,7 +35,7 @@
 #include <pcu_l1_if.h>
 #include <gprs_debug.h>
 #include <gprs_bssgp_pcu.h>
-#include <pcuif_proto.h>
+#include <osmocom/pcu/pcuif_proto.h>
 #include <bts.h>
 #include <tbf.h>