package/olsr: enable back on musl with headers >= 4.15

package/olsr: enable back on musl with headers >= 4.15

olsr has been disabled on musl three years ago by commit
4bca88c839e54a9140c06010666f5e034127b8b1.

Comments in https://github.com/OLSR/olsrd/issues/6 suggest that the
issue was due to olsr mixing kernel and userspace headers. This issue is
still open.

However, nowadays olsr builds fine on musl under those two conditions:
- musl >= 1.1.16 (i.e. with
  http://git.musl-libc.org/cgit/musl/commit/?id=04983f2272382af92eb8f8838964ff944fbb8258)
- kernels >= 4.15 (i.e. with
  https://github.com/torvalds/linux/commit/c0bace798436bca0fdc221ff61143f1376a9c3de)

So enable it back on musl with kernels >= 4.15.

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