sh: No need to explicitly include <linux/rwlock.h>.

Since <linux/spinlock.h> already includes <linux/rwlock.h>, and the
latter file will warn about not having included the former file
anyway, there is no value in including rwlock.h explicitly.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>