building: remove libvirt_dbus.syms from EXTRA_DIST

commit 2223ea98 removes src/libvirt_dbus.syms, but it forgets
to remove it from EXTRA_DIST. It will cause 'make dist' failed.

src/Makefile.am

@@ -1230,7 +1230,6 @@ EXTRA_DIST += \
   libvirt_driver_modules.syms	\
   libvirt_daemon.syms		\
   libvirt_linux.syms		\
-  libvirt_dbus.syms		\
   libvirt_network.syms		\
   libvirt_nwfilter.syms		\
   libvirt_sasl.syms		\