buildroot

package/libnet: enable on musl configurations

Thanks to a patch taken from upstream
(https://github.com/sam-github/libnet/commit/ffd7fab744a9ad2893169a8fb6244074604d5d0d),
we can enable the libnet package on musl toolchain.

Adjusted the file paths manually so the patch applies to the packaged
libnet sources.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
[Thomas: allow the package to be selected with the musl C library.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>