efivar: add musl libc compatibility

Backport patch to get efivar working with musl.

Upstream issue:
https://github.com/rhboot/efivar/issues/202



After commit meta-oe/1582f81805ee3114bc1a44bd5cf52d21f96702ca
fwupd gives an error when trying to build with musl because
efivar is not compatible. This fixes the issue.

Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>