Bug 1766510: fix windows 7 taskbar pinning code in mingw builds r=rkraesig

Differential Revision: https://phabricator.services.mozilla.com/D144826

browser/components/shell/nsWindowsShellService.cpp

@@ -3,6 +3,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#define UNICODE
+
 #include "nsWindowsShellService.h"
 
 #include "BinaryPath.h"