Hide private StretchBltModeChanger class in unnamed namespace

No real changes, just put this non-wx-prefixed class in an unnamed
namespace to avoid any clashes with user-defined classes (which would be
possible when using wx as a static library).

src/msw/dc.cpp

@@ -404,6 +404,9 @@ private:
     wxDECLARE_NO_COPY_CLASS(wxBrushAttrsSetter);
 };
 
+namespace
+{
+
 // this class sets the stretch blit mode to COLORONCOLOR during its lifetime
 class StretchBltModeChanger
 {
@@ -434,6 +437,8 @@ private:
     wxDECLARE_NO_COPY_CLASS(StretchBltModeChanger);
 };
 
+} // anonymous namespace
+
 // ===========================================================================
 // implementation
 // ===========================================================================