Vlv2TbltDevicePkg/PlatformGopPolicy: include UefiBootServicesTableLib.h

Vlv2TbltDevicePkg/PlatformGopPolicy: include UefiBootServicesTableLib.h

gBS is already defined in
MdePkg/Library/UefiBootServicesTableLib/UefiBootServicesTableLib.c

Remove the declaration of gBS and include the proper header file to
access the variable.

Suggested-by: David Wei <david.wei@intel.com>
Cc: David Wei <david.wei@intel.com>
Cc: "Wu, Hao A" <hao.a.wu@intel.com>
Cc: "Lu, ShifeiX A" <shifeix.a.lu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>