meson: Install missing example files
Install the example files from contrib/spi/, to match makefiles.
Reviewed-by: Tristan Partin <tristan@neon.tech>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/
b018b577-38a2-49c6-8727-
adfb577de317@eisentraut.org