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/configure.ac b/configure.ac
index 022a7f7..dcde2ed 100644
--- a/configure.ac
+++ b/configure.ac
@@ -83,6 +83,7 @@
 AC_SUBST(STD_DEFINES_AND_INCLUDES)
 
 AC_OUTPUT(
+    include/Makefile
     src/Makefile
     examples/Makefile
     tests/Makefile