windows: add missing DllImport attribute to LocalFree (#923)

Accidentally left off the `[DllImport]` attribute when adding this
`extern`.. now it'll actually point to the correct function in the DLL.

Author: mjcheetham

src/shared/Core/Interop/Windows/Native/Kernel32.cs

@@ -249,6 +249,7 @@ public static extern bool SetConsoleMode(
         /// <para/>
         /// To get extended error information, call GetLastError.
         /// </returns>
+        [DllImport(LibraryName, CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Unicode, SetLastError = true)]
         public static extern IntPtr LocalFree(IntPtr ptr);
     }