Fix ABI break by moving PROCSIG_SLOTSYNC_MESSAGE in ProcSignalReason

Fix ABI break by moving PROCSIG_SLOTSYNC_MESSAGE in ProcSignalReason

Commit 58c1188a3ea added PROCSIG_SLOTSYNC_MESSAGE in the middle of
enum ProcSignalReason, breaking the ABI.

Fix this by moving PROCSIG_SLOTSYNC_MESSAGE to the end of the enum,
to restore ordering.

Per buildfarm member crake.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwH_AAbtsiYDJt65N7_4PJ0CgOJmBEaCq68e5_tcuG_vXw@mail.gmail.com
Backpatch-through: 18 only