dummycon: only build module if there are users

Dummycon is used as a fallback conswitchp for vgacon and fbcon
in the VT code, and there are no references to it if all three
are disabled, so just leave it out of the kernel image for
configurations without those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>