doxygen: Remove the tag file on uninstall as well

Hopefully last patch:

ERROR: files left in build directory after distclean:
./doc/libosmocore.tag
./doc/libosmovty.tag
./doc/libosmogsm.tag
./doc/libosmocodec.tag
Makefile:800: recipe for target 'distcleancheck' failed
diff --git a/Makefile.am b/Makefile.am
index 8c28b81..185127e 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -50,7 +50,7 @@
 uninstall-hook:
 	cd $(DESTDIR)$(htmldir) && rm -rf {core,gsm,vty,codec}
 
-DX_CLEAN = doc/{core,gsm,vty,codec}/html/search/* doc/{core,gsm,vty,codec}/{html,latex}/* doc/html.tar doc/{core,gsm,vty,codec}/doxygen_sqlite3.db
+DX_CLEAN = doc/{core,gsm,vty,codec}/html/search/* doc/{core,gsm,vty,codec}/{html,latex}/* doc/html.tar doc/{core,gsm,vty,codec}/doxygen_sqlite3.db doc/*.tag
 endif
 
 MOSTLYCLEANFILES = $(DX_CLEAN)