Fix executor/spi.h to follow our usual conventions for include files, ie,

Fix executor/spi.h to follow our usual conventions for include files, ie,
not include postgres.h nor anything else it doesn't directly need.  Add
#includes to calling files as needed to compensate.  Per my proposal of
yesterday.

This should be noted as a source code change in the 8.4 release notes,
since it's likely to require changes in add-on modules.