Improve InitShmemAccess() prototype

Improve InitShmemAccess() prototype

The code comment said, 'the argument should be declared "PGShmemHeader
*seghdr", but we use void to avoid having to include ipc.h in
shmem.h.'  We can achieve the original goal with a struct forward
declaration.  (ipc.h was also not the correct header file.)

Discussion: https://www.postgresql.org/message-id/flat/cnthxg2eekacrejyeonuhiaezc7vd7o2uowlsbenxqfkjwgvwj@qgzu6eoqrglb