watchdog: constify watchdog_set_device() parameter

src/shared/watchdog.c

@@ -88,7 +88,7 @@ static int open_watchdog(void) {
         return update_timeout();
 }
 
-int watchdog_set_device(char *path) {
+int watchdog_set_device(const char *path) {
         int r;
 
         r = free_and_strdup(&watchdog_device, path);

src/shared/watchdog.h

@@ -6,7 +6,7 @@
 #include "time-util.h"
 #include "util.h"
 
-int watchdog_set_device(char *path);
+int watchdog_set_device(const char *path);
 int watchdog_setup(usec_t timeout);
 int watchdog_ping(void);
 void watchdog_close(bool disarm);