staging: r8188eu: Fix missing header

Commit 2397c6e0 entitled "staging: r8188eu:
Remove wrappers around vmalloc and vzalloc" and
commit: 03bd6aea entitled "staging: r8188eu:
Remove wrappers around vfree" failed to add the header file needed
to provide vzalloc and vfree.

This problem was reported by the kbuild test robot.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>