Staging: speakup: Remove 'extern' keywords from .h prototypes

Addresses checkpatch.pl check:
CHECK: Extern prototypes should be avoided in .h files

Removes the 'extern' keyword from function prototypes

Signed-off-by: Katie Dunne <kdunne@mail.ccsf.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>