[gn] port c4a0197 more

c4a0197 did a great job updating the GN build files, but it
missed this one detail.

Author: nico

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -50,6 +50,7 @@ write_lit_config("lit_site_cfg") {
     "CLANG_DEFAULT_CXX_STDLIB=",  # Empty string means "default value" here.
     "CLANG_DEFAULT_PIE_ON_LINUX=0",
     "CLANG_ENABLE_CIR=0",
+    "CLANG_ENABLE_OBJC_REWRITER=1",  # FIXME: flag?
     "CLANG_PLUGIN_SUPPORT=0",  # FIXME: Analysis/plugins need global -fPIC
     "CURRENT_TOOLS_DIR=" + rebase_path("$root_out_dir/bin", dir),
     "CLANG_VENDOR_UTI=org.llvm.clang",