openrisc/delay: Add include to fix symbol not declared warning

Add asm/timex.h include for read_current_timer prototype.

Sparse reporting the following warning:

    arch/openrisc/lib/delay.c:23:5: warning: symbol 'read_current_timer' was not declared. Should it be static?

Signed-off-by: Stafford Horne <shorne@gmail.com>