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: cb095afd ("pstore: Centralize init/exit routines")
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -793,7 +793,7 @@ static void pstore_timefunc(struct timer_list *unused)
 			  jiffies + msecs_to_jiffies(pstore_update_ms));
 }
 
-void __init pstore_choose_compression(void)
+static void __init pstore_choose_compression(void)
 {
 	const struct pstore_zbackend *step;