rtl8192cu: fix build error (vmalloc/vfree undefined)

On the ARM system, a build fails due to missing include.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>