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
1 file changed