pstore: Make sure CONFIG_PSTORE_PMSG selects CONFIG_RT_MUTEXES

In commit 76d62f2 ("pstore: Switch pmsg_lock to an rt_mutex
to avoid priority inversion") I changed a lock to an rt_mutex.

However, its possible that CONFIG_RT_MUTEXES is not enabled,
which then results in a build failure, as the 0day bot detected:
  https://lore.kernel.org/linux-mm/[email protected]/

Thus this patch changes CONFIG_PSTORE_PMSG to select
CONFIG_RT_MUTEXES, which ensures the build will not fail.

Cc: Wei Wang <[email protected]>
Cc: Midas Chien<[email protected]>
Cc: Connor O'Brien <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: [email protected]
Fixes: 76d62f2 ("pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: John Stultz <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

fs/pstore/Kconfig

@@ -126,6 +126,7 @@ config PSTORE_CONSOLE
 config PSTORE_PMSG
 	bool "Log user space messages"
 	depends on PSTORE
+	select RT_MUTEXES
 	help
 	  When the option is enabled, pstore will export a character
 	  interface /dev/pmsg0 to log user space messages. On reboot