gpu: nvgpu: Fix build when gk20a is not enabled

gpu: nvgpu: Fix build when gk20a is not enabled

This patch adds a missing include to the public gk20a header. The
header used error code definitions (-ENOSYS) despite errno.h was
not included.

Change-Id: Iaa389d6ea0f2fc66b911984fa863bf3327636c47
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-on: http://git-master/r/397589
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>