package/efivar: fix build with musl libc

Backport upstream patch to fix build with musl libc.
This patch is only a requirement since efivar 38 and was applied
upstream shortly after the 38 version tag.

Fixes:
http://autobuild.buildroot.net/results/c49d894b109d68e2624074eab8b939fefa3b42ef/



Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>