Fix MinGW and VC8 compilation error

3a8ad952 introduced compilation failures with (some versions of) MinGW
and VC8 because their winnls.h apparently doesn't declare
SetThreadUILanguage(). Just declare the function ourselves to fix it.

src/common/intl.cpp

@@ -50,6 +50,8 @@
 
 #ifdef __WIN32__
     #include "wx/msw/private.h"
+    // Some compilers' winnls.h header doesn't declare this function:
+    extern "C" WINBASEAPI LANGID WINAPI SetThreadUILanguage(LANGID LangId);
 #endif
 
 #include "wx/file.h"