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>

arch/openrisc/lib/delay.c

@@ -15,6 +15,7 @@
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/init.h>
+#include <linux/timex.h>
 #include <asm/param.h>
 #include <asm/delay.h>
 #include <asm/timex.h>