speakup: replace utils' u_char with unsigned char

drivers/accessibility/speakup/utils.h will be used to compile host tool
to generate metadata.

"u_char" is a non-standard type, which is defined to "unsigned char"
on glibc but not defined by some libc, e.g. musl.

Let's replace "u_char" with "unsigned char"

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/b75743026aaee2d81efe3d7f2e8fa47f7d0b8ea7.1665736571.git.congdanhqx@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>