staging: vchip_shim: Remove unneeded stddef.h include

Building on openrisc musl toolchain this causes the allyesconfig build
to fail.

  drivers/staging/vc04_services/interface/vchiq_arm/vchiq_shim.c:42:20:
  fatal error: stddef.h: No such file or directory

Removing this causes no issues with the build.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>