tests: Add forgotten backslash

While introducing virtestmock.la, I've forgotten to add '\' at
the end of one line leaving our Makefile.am mangled. Fortunately,
the only thing that comes after is '$(NULL)' so nothing is
terribly broken.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -447,7 +447,7 @@ endif WITH_DBUS
 if WITH_LINUX
 test_libraries += virusbmock.la \
 		virnetdevbandwidthmock.la \
-		virtestmock.la
+		virtestmock.la \
 		$(NULL)
 endif WITH_LINUX