common: remove duplicate typedef for uchar

With commit 37db55b7 ("linux/types.h: fix typo unchar") we have a
duplicate typedef for uchar. As linux/types.h is included in common.h we
don't need another typedef for uchar there.

Fixes: 37db55b7 ("linux/types.h: fix typo unchar")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>