net: zynq: Fix sparse warnings in gem

Add missing header.

Warnings:
drivers/net/zynq_gem.c:491:5: warning: symbol 'zynq_gem_initialize' was not declared. Should it be static?
drivers/net/zynq_gem.c:542:5: warning: symbol 'zynq_gem_of_init' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>