Fix / suppress new unused warnings for mingw+clang

Bug: 69933068
Test: mmma system/core
Change-Id: I089166a979d3d8c5ada38a7745d507b555048499
diff --git a/utf8.cpp b/utf8.cpp
index 5984fb0..adb46d0 100644
--- a/utf8.cpp
+++ b/utf8.cpp
@@ -195,7 +195,7 @@
   return _wfopen(name_utf16.c_str(), mode_utf16.c_str());
 }
 
-int mkdir(const char* name, mode_t mode) {
+int mkdir(const char* name, mode_t) {
   std::wstring name_utf16;
   if (!UTF8PathToWindowsLongPath(name, &name_utf16)) {
     return -1;