speakup: use SPKDIR=$(src) to specify the source directory

Commit e68a558f ("speakup: Fix building as extmod") was intended
to support building this as an external module.

Since commit b1992c37 ("kbuild: use $(src) instead of
$(srctree)/$(src) for source directory"), $(src) consistently points
to the source directory, regardless of whether it is compiled as an
external module or not.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>