pstore: Make pstore_choose_compression() static

pstore: Make pstore_choose_compression() static

The pstore_choose_compression() function is not exported so make it
static to avoid the following sparse warning:

fs/pstore/platform.c:796:13: warning: symbol 'pstore_choose_compression' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20191016123317.3154-1-ben.dooks@codethink.co.uk
Fixes: aa60f34125bd ("pstore: Centralize init/exit routines")
Signed-off-by: Kees Cook <keescook@chromium.org>