When including <glib.h>, it will always define G_LOG_DOMAIN if it
is not yet defined.
Usually we want to include "nm-default.h" as very first header. In that
case, <glib.h> is not yet included. Then the previous check #error works
well.
However, if we include "nm-default.h" in sources generated by
glib-mkenums, then the generator first already includes <glib.h>,
and thus defines G_LOG_DOMAIN. It does so for "libnm-core/nm-core-enum-types.c"
and "libnm/nm-enum-types.c", where the #error would not trigger.
But we will also include "nm-default.h" for "libnm-core/tests/nm-core-tests-enum-types.c".
That will start triggering this #error.
While in general we want to include "nm-default.h" first, we also need
to support cases where <glib.h> gets included first. Thus this error is
not useful. Remove it.
(cherry picked from commit 42fa8f3d27)
a1f3cebbec