[gn] Include the missing BUILD.gn file for libcxxabi includes

[phosek]

Summary

This was omitted in r359805.

llvm/trunk/utils/gn/secondary/libcxx/include/BUILD.gn

if (target_os == "win") {
         "support/win32/locale_win32.h",
       ]
     }
   }
   outputs = [
     "$clang_resource_dir/include/c++/v1/{{source_target_relative}}",
   ]
 }
-
-copy("cxxabi_include") {
-  sources = [
-    "//libcxxabi/include/__cxxabi_config.h",  # ignore for sync script
-    "//libcxxabi/include/cxxabi.h",  # ignore for sync script
-  ]
-  outputs = [
-    "$clang_resource_dir/include/c++/v1/{{source_target_relative}}",
-  ]
-}

llvm/trunk/utils/gn/secondary/libcxxabi/include/BUILD.gn

+import("//clang/resource_dir.gni")
+
+copy("include") {
+  sources = [
+    "__cxxabi_config.h",
+    "cxxabi.h",
+  ]
+  outputs = [
+    "$clang_resource_dir/include/c++/v1/{{source_target_relative}}",
+  ]
+}