fbcode_builder: add a license header to FBBuildOptions.cmake

Summary:
Add a license header to satisfy the open source linter. Use the same
header the other .cmake files have.

Reviewed By: mhlakhani

Differential Revision: D17404782

fbshipit-source-id: 66679d72c9e680f8bb8b27869e981a046b3520cf

build/fbcode_builder/CMake/FBBuildOptions.cmake

+# Copyright (c) Facebook, Inc. and its affiliates.
+
 function (fb_activate_static_library_option)
   option(USE_STATIC_DEPS_ON_UNIX
     "If enabled, use static dependencies on unix systems. This is generally discouraged."