fix(freebsd): include semaphore_impl.h for FreeBSD platform

Fix issue #174: FreeBSD was excluded from including the POSIX semaphore
implementation header. The previous conditional compilation had an empty
branch for LIBIPC_OS_FREEBSD, causing 'no member named sync in namespace
ipc::detail' errors.

FreeBSD supports POSIX semaphore APIs (sem_open, sem_wait, sem_post, etc.),
so it should use the same semaphore_impl.h as Linux and QNX.

Author: mutouyun

src/libipc/sync/semaphore.cpp

@@ -7,8 +7,7 @@
 #include "libipc/platform/detail.h"
 #if defined(LIBIPC_OS_WIN)
 #include "libipc/platform/win/semaphore.h"
-#elif defined(LIBIPC_OS_QNX) || defined(LIBIPC_OS_FREEBSD)
-#elif defined(LIBIPC_OS_LINUX) || defined(LIBIPC_OS_QNX)
+#elif defined(LIBIPC_OS_LINUX) || defined(LIBIPC_OS_QNX) || defined(LIBIPC_OS_FREEBSD)
 #include "libipc/platform/posix/semaphore_impl.h"
 #else/*IPC_OS*/
 #   error "Unsupported platform."