Fix harmless warning from g++ 4.8 in C11 mode.

g++ 4.8 recognizes "cpu" in WX_CC_MANIFEST expansion as a user-defined literal
suffix, add spaces around it to prevent this from happening.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71506 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/defs.h

@@ -3282,7 +3282,7 @@ typedef const void* WXWidget;
     "/manifestdependency:\"type='win32'         \
      name='Microsoft.Windows.Common-Controls'   \
      version='6.0.0.0'                          \
-     processorArchitecture='"cpu"'              \
+     processorArchitecture='" cpu "'            \
      publicKeyToken='6595b64144ccf1df'          \
      language='*'\""