Fix parallel build
When trying to build the library with -j8 -l8, it fails with the following error because the tests do not explicitly depend on the generated enum types, causing a race.
../libsignon-glib/signon-errors.h:31:10: fatal error: libsignon-glib/signon-enum-types.h: No such file or directory
#include <libsignon-glib/signon-enum-types.h>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~