avoid including rel.h in execnodes.h

This simple patch moves two struct declarations (Trigger and
TriggerDesc) from rel.h into a new file, reltrigger.h. The benefit is
that execnodes.h only needs to include the latter. Since execnodes.h is
very widely included, this change means there are less files that
indirectly include rel.h now, which is a good thing because rel.h
includes a ton of other files. (Of course, rel.h itself needs to
include the new header).

I also included rel.h in spi.h, because it was previously indirectly
included via execnodes.h and with this patch it would no longer be,
which is a problem because it'd cause external code to fail to compile.

--
Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>