atk/Makefile.am: Stop reading atk.def

Since 41442d82 we no longer generate atk.def, but we were still reading
it when linking libatk on MinGW.  This was causing a build failure:

      CC       atkwindow.lo
      CC       atk-enum-types.lo
    /usr/bin/i686-w64-mingw32-windres atk.rc atk-win32-res.o
      CCLD     libatk-1.0.la
    libtool: link: symbol file `atk.def' does not exist
    make[3]: *** [libatk-1.0.la] Error 1

https://bugzilla.gnome.org/show_bug.cgi?id=730859