UTF8: Fix MSVC builds

MSVC does not support variable length arrays.  Substitute a std::vector
instead.

common/utf8.cpp

@@ -26,6 +26,7 @@
 #include <ki_exception.h>
 #include <wx/strconv.h>
 #include <wx/buffer.h>
+#include <vector>
 
 /* THROW_IO_ERROR needs this, but it includes this file, so until some
     factoring of THROW_IO_ERROR into a separate header, defer and use the asserts.
@@ -203,10 +204,9 @@ UTF8::UTF8( const wchar_t* txt )
 {
     try
     {
-        size_t len = wcslen( txt ) * 4 + 1;
-        char temp[len];
-        wxConvUTF8.WC2MB( temp, txt, len );
-        m_s.assign( temp );
+        std::vector< char > temp( wcslen( txt ) * 4 + 1 );
+        wxConvUTF8.WC2MB( temp.data(), txt, temp.size() );
+        m_s.assign( temp.data() );
     }
     catch(...)
     {