GameUI: use V_isspace instead

This should be the fastest version available without any additional locale awareness.

src/gameui/IConsole.cpp

@@ -664,7 +664,7 @@ bool CConsole::RunAutoComplete(void)
         {
             const char c = m_inputTextBuf[i];
 
-            if (c == '\0' || isspace(c))
+            if (c == '\0' || V_isspace(c))
             {
                 break;
             }