debian: fix: add pcuif_proto.h to osmo-pcu.install

Following 68fc12775fac03e5e26a008dfa68c92498bfb772
'Install the pcuif_proto.h header file'
we need to add pcuif_proto.h to the debian install file.

Change-Id: Ib8e185900826baadcc96fcde1491903dbaf85f8b
diff --git a/debian/osmo-pcu.install b/debian/osmo-pcu.install
index 1f9a8e0..768719c 100644
--- a/debian/osmo-pcu.install
+++ b/debian/osmo-pcu.install
@@ -1,2 +1,3 @@
 etc/osmocom/osmo-pcu.cfg
 usr/bin/osmo-pcu
+usr/include/osmocom/pcu/pcuif_proto.h