workaround SR-12939 (#826)

Sources/CGRPCZlib/empty.c

@@ -13,3 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+// Xcode's Archive builds with Xcode's Package support struggle with empty .c files
+// (https://bugs.swift.org/browse/SR-12939).
+void CGRPCZlib_i_do_nothing_just_working_around_a_darwin_toolchain_bug(void) {}