lib: use `prefix` for yang get prefix wrapper

lib: use `prefix` for yang get prefix wrapper

This change fixes a static analyzer warning and should also make us
safer when using this function. At the moment the code that triggered
the warning is the only one that uses this function.

Passing anything other than `struct prefix` to `str2prefix` function is
dangerous, because the structure might be smaller than expected and we
might have an buffer overflow.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>