deb: fix build by adding libosmogb doc package

Change-Id: I3fe45feaee4033c17e25c4c0552d54a81d9b44f5
Fixes: OS#2340
diff --git a/debian/rules b/debian/rules
index e173510..2399b01 100755
--- a/debian/rules
+++ b/debian/rules
@@ -53,5 +53,6 @@
 	$(RM) -r doc/codec/
 	$(RM) -r doc/core/
 	$(RM) -r doc/gsm/
+	$(RM) -r doc/gb/
 	$(RM) -r doc/vty/html/
 	$(RM) -r doc/vty/latex/