--disable-plugin-gspell option don't change org.xfce.mousepad.gschema.xml accordingly

if configured mousepad with --disable-plugin-gspell, org.xfce.mousepad.plugins.gspell.gschema.xml will not be generated. but org.xfce.mousepad.gschema.xml still refer org.xfce.mousepad.plugins.

which make command 'glib-compile-schemas /usr/share/glib-2.0/schemas' report error: Warning: undefined reference to <schema id='org.xfce.mousepad.plugins'/>