Revert "building: remove libvirt_dbus.syms from EXTRA_DIST"

This reverts commit 372a14c6.

We shoul not have cherry-picked 28ae4ff0 without also cherry-picking
2223ea98, but the latter is too complex for a stable branch.

src/Makefile.am

@@ -1232,6 +1232,7 @@ EXTRA_DIST += \
   libvirt_driver_modules.syms	\
   libvirt_daemon.syms		\
   libvirt_linux.syms		\
+  libvirt_dbus.syms		\
   libvirt_network.syms		\
   libvirt_nwfilter.syms		\
   libvirt_sasl.syms		\