package/sslh: bump to version 1.21c

package/sslh: bump to version 1.21c

- Drop patch (already in version)
- libbsd is an optional dependency since version 1.21b and
  https://github.com/yrutschle/sslh/commit/0380a4309f826062a4ab772e65077c0566d8fbd4
- libconfig is optional since version 1.21c and
  https://github.com/yrutschle/sslh/commit/2c93a015ea1d261a934df00285691522e6a6d211
- Update indentation in hash file (two spaces)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>