panel-plugin/title/windowck-title.c: fix memory leak on title change

It seems the memory is allocated on every title change and never gets
freed. Glib says to use g_malloc() with g_free() (used below) and it
seems switching to g_malloc() fixes the memory leak.

Refs https://github.com/cedl38/xfce4-windowck-plugin/issues/30



Signed-off-by: Mateusz Gozdek <mgozdekof@gmail.com>