musl: workaround kernel if_ether.h header conflict

musl: workaround kernel if_ether.h header conflict

Add a patch that defines _LINUX_IF_ETHER_H in netinet/if_ether.h to avoid the
conflicting kernel if_ether.h.

Other musl distros (Alpine, Sabotage, Gentoo) patch[1] the kernel headers
instead. This is not practical in Buildroot, since we allow the user to choose
the kernel version.

This only fixes the normal case of including linux/if_ether.h (directly or
indirectly) after netinet/if_ether.h.

Fixes (tcpreplay):
http://autobuild.buildroot.net/results/a65/a65b27f9e288f94f041ced64313dbf55af47bf36/

[1] https://cgit.gentoo.org/proj/musl.git/diff/sys-kernel/linux-headers/files/libc-compat.h-prevent-redefinition-of-struct-ethhdr.patch?id=8eddda8072add075ebf56cf6d288bc1450d6b5f8

Cc: Rich Felker <dalias@aerifal.cx>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>