xfce4-session: exclude machine-host dependency with SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS

* this whole machine-host is very bad idea, all changes from postinst
  will be removed when netbase package is upgraded (it has CONFFILES
  set but most users won't know that this entry was appended by
  machine-host package)
* see:
  http://lists.openembedded.org/pipermail/openembedded-devel/2015-September/103271.html
* fixes:
ERROR: xfce4-session different signature for task do_package_write_ipk.sigdata between qemux86copy and qemux86
runtaskdeps changed from ['dbusdbus_1.8.20.bb.do_packagedata', 'opkg-utilsopkg-utils_git.bb.do_populate_sysroot:virtual:native', 'pseudopseudo_1.7.4.bb.do_populate_sysroot:virtual:native', 'upowerupower_0.9.14.bb.do_packagedata', 'xfce4-sessionmachine-host.bb.do_packagedata', 'xfce4-sessionxfce4-session_4.12.1.bb.do_package', 'xfce4-sessionxfce4-session_4.12.1.bb.do_packagedata', 'xorg-appiceauth_1.0.7.bb.do_packagedata', 'xorg-appxinit_1.3.4.bb.do_packagedata'] to ['dbusdbus_1.8.20.bb.do_...