pthreads: use simple Genode::Semaphore as backend

Instead of using Timed_semaphore, we can directly use the simpler
Genode::Semaphore for pthread's 'sem_t' type.

Ref #1333

repos/libports/src/lib/pthread/semaphore.cc

@@ -12,9 +12,8 @@
  * under the terms of the GNU General Public License version 2.
  */
 
+#include <base/env.h>
 #include <base/printf.h>
-#include <os/timed_semaphore.h>
-
 #include <semaphore.h>
 
 using namespace Genode;
@@ -25,9 +24,9 @@ extern "C" {
 	 * This class is named 'struct sem' because the 'sem_t' type is
 	 * defined as 'struct sem*' in 'semaphore.h'
 	 */
-	struct sem : Timed_semaphore
+	struct sem : Semaphore
 	{
-		sem(int value) : Timed_semaphore(value) { }
+		sem(int value) : Semaphore(value) { }
 	};