xkb: Print the xkbcomp path being executed when we fail to compile.

I don't know how many times I've had a broken server due to a bad
directory to xkbcomp, and only finding the whole path has shown me
where I went wrong.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

xkb/ddxLoad.c

@@ -179,8 +179,10 @@ RunXkbComp(xkbcomp_buffer_callback callback, void *userdata)
 #endif
             return xnfstrdup(keymap);
         }
-        else
+        else {
-            LogMessage(X_ERROR, "Error compiling keymap (%s)\n", keymap);
+            LogMessage(X_ERROR, "Error compiling keymap (%s) executing '%s'\n",
+                       keymap, buf);
+        }
 #ifdef WIN32
         /* remove the temporary file */
         unlink(tmpname);