Fix handling non-ASCII format strings in UTF-8 build

Constructing wxString from "char*" format string was wrong as it
interpreted it in the current locale encoding which could be different
from UTF-8.

(cherry picked from commit 5a9e433524020dc6b653183bdd338bb3647902a9)

src/common/string.cpp

@@ -1976,7 +1976,7 @@ int wxString::DoPrintfUtf8(const char *format, ...)
     va_list argptr;
     va_start(argptr, format);
 
-    int iLen = PrintfV(format, argptr);
+    int iLen = PrintfV(wxString::FromUTF8(format), argptr);
 
     va_end(argptr);