agp: Include "compat_ioctl.h" where necessary

Fix compiler warnings like

  drivers/char/agp/frontend.c:46:20: warning: no previous prototype for 'agp_find_mem_by_key' [-Wmissing-prototypes]
    46 | struct agp_memory *agp_find_mem_by_key(int key)

by including the compat_ioctl.h in the source file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201114645.15384-3-tzimmermann@suse.de