stm class: Select configfs

STM policy handling is basically configfs, I honestly don't know how we
ended up without a Kconfig dependency, but thanks to randconfig testing,
it's now caught.
Reported-by: NJim Davis <jim.epost@gmail.com>
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Reported-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NAlexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/stm/Kconfig

 config STM
 	tristate "System Trace Module devices"
+	select CONFIGFS_FS
 	help
 	  A System Trace Module (STM) is a device exporting data in System
 	  Trace Protocol (STP) format as defined by MIPI STP standards.