hvfb_getmem uses vzalloc, therefore vmalloc.h should be included.

Fixes commit d21987d7 ("video: hyperv:
hyperv_fb: Support deferred IO for Hyper-V frame buffer driver")
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106183941.9751-1-olaf@aepfle.de