Fix header inclusion order in xloginsert.c with lz4.h

Fix header inclusion order in xloginsert.c with lz4.h

Per project policy, all system and library headers need to be declared
in the backend code after "postgres.h" and before the internal headers,
but 4035cd5 broke this policy when adding support for LZ4 in
wal_compression.

Noticed while reviewing the patch to add support for zstd in this area.
This only impacts HEAD, so there is no need for a back-patch.