reboot: export symbol 'reboot_mode'

Some drivers like Qualcomm pm8941-pwrkey need to access 'reboot_mode'
for triggering reboot between cold and warm mode.  Export the symbol, so
that drivers built as module can still access the symbol.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Link: https://lore.kernel.org/r/20210714095850.27185-2-shawn.guo@linaro.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

kernel/reboot.c

@@ -32,6 +32,7 @@ EXPORT_SYMBOL(cad_pid);
 #define DEFAULT_REBOOT_MODE
 #endif
 enum reboot_mode reboot_mode DEFAULT_REBOOT_MODE;
+EXPORT_SYMBOL_GPL(reboot_mode);
 enum reboot_mode panic_reboot_mode = REBOOT_UNDEFINED;
 
 /*