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 c9cabf9)

tests/sys/fs/fusefs/setattr.cc

@@ -34,6 +34,7 @@ extern "C" {
 #include <sys/stat.h>
 
 #include <fcntl.h>
+#include <semaphore.h>
 }
 
 #include "mockfs.hh"