libcamera: Fix maybe-uninitialized error

The gcc used in my current buildroot (Version 12.3) errors out with
-Wmaybe-uninitialized. Fix that.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/libcamera/device_enumerator_sysfs.cpp

@@ -33,7 +33,7 @@ int DeviceEnumeratorSysfs::init()
 int DeviceEnumeratorSysfs::enumerate()
 {
 	struct dirent *ent;
-	DIR *dir;
+	DIR *dir = nullptr;
 
 	static const char * const sysfs_dirs[] = {
 		"/sys/subsystem/media/devices",