Explicitly include semaphore.h for struct _sem in fusefs setattr test

In libc++'s __threading_support header the semaphore.h header was
implicitly included, but from version 14 onwards, this is no longer the
case, resulting in compile errors:

tests/sys/fs/fusefs/setattr.cc:740:8: error: variable has incomplete type 'sem_t' (aka '_sem')

sem_t sem;
      ^

tests/sys/fs/fusefs/utils.hh:33:8: note: forward declaration of '_sem'
struct _sem;

^

MFC after: 3 days

(cherry picked from commit c9cabf9aa6feb542776333fe5e915a3464f89e3c)