efibootmgr: update compilation with musl

Since the commit 005b6aba
(efivar: add musl libc compatibility) efibootmgr compiles with
musl too. Update the variable to take that into account.

Signed-off-by: Marta Rybczynska <marta.rybczynska@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>