staging: clocking-wizard: Include clk.h

This clock provider uses the consumer API, so include clk.h
explicitly.

Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>