switchroot: Fix building with musl libc

switchroot: Fix building with musl libc

POSIX and GNU define conflicting versions of `strerror_r`.  The GNU
version returns the string but doesn't necessilary write into buf.
The POSIX version writes into buf and returns the length but doesn't
necessilary append a terminate the string with a NUL if it's too long
to fit in buf.

This commit fixes building ostree-prepare-root with musl libc.  The
stripped static build with musl on my machine is 30K vs. 724K with glibc
static and 11K with glibc shared.

Closes: #477
Approved by: cgwalters