vcl: use ref to const while iterating over palette entries

Change-Id: I2a1b36d9f4bb17f8b60af86a48bc9ec84588a32b
Reviewed-on: https://gerrit.libreoffice.org/64749
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

vcl/win/gdi/gdiimpl.cxx

@@ -211,7 +211,7 @@ bool IsDitherColor(BYTE nRed, BYTE nGreen, BYTE nBlue)
 
 bool IsPaletteColor(BYTE nRed, BYTE nGreen, BYTE nBlue)
 {
-    static PALETTEENTRY aImplSalSysPalEntryAry[] =
+    static const PALETTEENTRY aImplSalSysPalEntryAry[] =
     {
     {    0,    0,    0, 0 },
     {    0,    0, 0x80, 0 },
@@ -231,7 +231,7 @@ bool IsPaletteColor(BYTE nRed, BYTE nGreen, BYTE nBlue)
     { 0xFF, 0xFF, 0xFF, 0 }
     };
 
-    for (auto& rPalEntry : aImplSalSysPalEntryAry)
+    for (const auto& rPalEntry : aImplSalSysPalEntryAry)
     {
         if(rPalEntry.peRed == nRed &&
            rPalEntry.peGreen == nGreen &&