src/Makefile.am: Add forgotten backslash

As I've pushed 5892944f I haven't noticed one small nitpick.
There was this backslash missing on the line 1231 in the
enumeration of libraries to be added to vbox storage driver. This
resulted in nondeterministic build which sometimes succeeded and
sometimes failed.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/Makefile.am

@@ -1228,7 +1228,7 @@ libvirt_driver_vbox_storage_impl_la_CFLAGS =	\
 libvirt_driver_vbox_storage_impl_la_LDFLAGS = $(AM_LDFLAGS)
 libvirt_driver_vbox_storage_impl_la_LIBADD = $(DLOPEN_LIBS)		\
 		$(MSCOM_LIBS)					\
-		$(LIBXML_LIBS)
+		$(LIBXML_LIBS)					\
 		libvirt_driver_vbox_impl.la
 libvirt_driver_vbox_storage_impl_la_SOURCES = $(VBOX_STORAGE_DRIVER_SOURCES)
 endif WITH_VBOX