C++11 requires a space between adjacent string literals. This change prevents recent GCC to emit a warning every time wx/defs.h is included.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@74127 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775