Fix some OUString construction error with VS2013

A bit unclear to me whether the code really did what was intended with
earlier VS versions either.

VS2013 said: error C2664: 'rtl::OUString::OUString(const sal_uInt32
*,sal_Int32)' : cannot convert argument 1 from 'const char *' to
'const rtl::OUString &'

Reason: cannot convert from 'const char *' to 'const rtl::OUString'

No constructor could take the source type, or constructor overload
resolution was ambiguous

Change-Id: Ie23d8382515064062a3ed46418d9d4a977b35d28

vcl/win/source/window/salframe.cxx

@@ -4441,7 +4441,7 @@ static int ImplMenuChar( HWND, WPARAM wParam, LPARAM lParam )
 {
     int nRet = MNC_IGNORE;
     HMENU hMenu = (HMENU) lParam;
-    OUString aMnemonic( "&" + (sal_Unicode) LOWORD(wParam) );
+    OUString aMnemonic( "&" + OUString((sal_Unicode) LOWORD(wParam)) );
     aMnemonic = aMnemonic.toAsciiLowerCase();   // we only have ascii mnemonics
 
     // search the mnemonic in the current menu