libosmocoding: fix .deb building

dpkg-buildpackage fails due to missing file descriptions - add necessary
.install and .doc-base files to fix it.

Change-Id: I5fb7e813c0860a3b5037e805deb84f9bf649ffa3
diff --git a/debian/libosmocoding-doc.install b/debian/libosmocoding-doc.install
new file mode 100644
index 0000000..278e3b5
--- /dev/null
+++ b/debian/libosmocoding-doc.install
@@ -0,0 +1 @@
+usr/share/doc/libosmocore/coding/