pstore: Fix Kconfig dependencies for apei->pstore

Geert Uytterhoeven ran a dependency checker which kicked out this warning:

+ warning: (ACPI_APEI) selects PSTORE which has unmet direct dependencies (MISC_FILESYSTEMS):  => N/A

Randy confirmed that the fix was to "select MISC_FILESYSTEMS" too.

Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>