busybox: allow build with musl

The musl C library does not yet implemented the getpwent_r() function,
needed by the Busybox password code when CONFIG_USE_BB_PWD_GRP is
disabled. So we enable it when the musl C library is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>