Stop casting away constness on dmx_extension_name

Fixes gcc warning:
dmx.c: In function 'find_display':
dmx.c:78:1: warning: cast discards qualifiers from pointer target type

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

src/dmx.c

@@ -76,7 +76,7 @@ static /* const */ XExtensionHooks dmx_extension_hooks = {
 };
 
 static XEXT_GENERATE_FIND_DISPLAY(find_display, dmx_extension_info,
-                                  (char *)dmx_extension_name,
+                                  dmx_extension_name,
                                   &dmx_extension_hooks,
                                   0, NULL)