deb: fix build by adding libosmogb doc package

Change-Id: I3fe45feaee4033c17e25c4c0552d54a81d9b44f5
Fixes: OS#2340
diff --git a/debian/libosmogb-doc.install b/debian/libosmogb-doc.install
new file mode 100644
index 0000000..957c82c
--- /dev/null
+++ b/debian/libosmogb-doc.install
@@ -0,0 +1 @@
+usr/share/doc/libosmocore/gb/