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>

drivers/acpi/apei/Kconfig

 config ACPI_APEI
 	bool "ACPI Platform Error Interface (APEI)"
+	select MISC_FILESYSTEMS
 	select PSTORE
 	depends on X86
 	help