pgsql: Silence compiler warnings for unsupported compression methods.

Silence compiler warnings for unsupported compression methods.

wrasse, at least, moans about the lack of any "return" statement
in these functions.  You'd think pretty much everything would
know that exit(1) doesn't return, but evidently not.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c0d1c641cbe433d1b6304bc1e3a2d8cd38b9a8e5

Modified Files
--------------
src/bin/pg_basebackup/bbstreamer_gzip.c | 2 ++
src/bin/pg_basebackup/bbstreamer_lz4.c  | 2 ++
src/bin/pg_basebackup/bbstreamer_zstd.c | 2 ++
3 files changed, 6 insertions(+)