tests: Expose test's fake_header.h so py_cc_toolchain tests can use it. (#1388)

Newer Bazel versions default to not exporting files by default; this
explicitly exports the file so it can be referenced.

Author: rickeylev

tests/cc/BUILD.bazel

@@ -19,6 +19,8 @@ load(":fake_cc_toolchain_config.bzl", "fake_cc_toolchain_config")
 
 package(default_visibility = ["//:__subpackages__"])
 
+exports_files(["fake_header.h"])
+
 toolchain(
     name = "fake_py_cc_toolchain",
     tags = PREVENT_IMPLICIT_BUILDING_TAGS,