strtol doesn't need a empty string, NULL is good enough.

Fixes:
evdev.c: In function 'EvdevInitButtonMapping':
evdev.c:1659:25: warning: initialization discards 'const' qualifier from
pointer target type [enabled by default]

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>
(cherry picked from commit 61faf2e6e7)

src/evdev.c

@@ -1660,7 +1660,7 @@ EvdevInitButtonMapping(InputInfoPtr pInfo)
     /* Check for user-defined button mapping */
     if ((mapping = xf86CheckStrOption(pInfo->options, "ButtonMapping", NULL)))
     {
-        char    *map, *s = " ";
+        char    *map, *s = NULL;
         int     btn = 0;
 
         xf86IDrvMsg(pInfo, X_CONFIG, "ButtonMapping '%s'\n", mapping);