drivers/gpu: arm/mali400: Fix build issue

The following statement doesn't have consistent behaviour on all machines:
$(wildcard $(src)/linux/license/gpl/*)
On some it actually returns the name of the GPL header file, on others it is a blank string.
Since the driver is in-tree, we can assume GPL compliance.

Used patch: https://lists.96boards.org/pipermail/dev/2015-April/000216.html