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
1 file changed