pgsql: Move extern declarations for EXEC_BACKEND to header files

Move extern declarations for EXEC_BACKEND to header files

This fixes warnings from -Wmissing-variable-declarations (not yet part
of the standard warning options) under EXEC_BACKEND.  The
NON_EXEC_STATIC variables need a suitable declaration in a header file
under EXEC_BACKEND.

Also fix the inconsistent application of the volatile qualifier for
PMSignalState, which was revealed by this change.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d3cc5ffe81f64c6418ba9b18a9db32392f8027e6

Modified Files
--------------
src/backend/postmaster/launch_backend.c | 12 +-----------
src/include/postmaster/postmaster.h     |  4 ++++
src/include/postmaster/syslogger.h      |  4 ++++
src/include/storage/pmsignal.h          |  4 ++++
src/include/storage/proc.h              |  5 +++++
5 files changed, 18 insertions(+), 11 deletions(-)