xffm head: include errno.h please ^^

[Danny Milosavljevic]

Hi edscott,

the following happens: 
toolbar.c: In function `tb_drag_data':
toolbar.c:331: `EINVAL' undeclared (first use in this function)
toolbar.c:331: (Each undeclared identifier is reported only once
toolbar.c:331: for each function it appears in.)

please include errno.h :)

Thanks :)