threads: use explicit constructors where possible

threads/barrier.hpp

@@ -22,7 +22,7 @@
 namespace util { namespace threads {
     class barrier {
     public:
-        barrier (unsigned count);
+        explicit barrier (unsigned count);
         ~barrier ();
 
         void wait (void);

threads/semaphore.hpp

@@ -22,7 +22,7 @@
 namespace util { namespace threads {
     class semaphore {
     public:
-        semaphore (unsigned value = 0);
+        explicit semaphore (unsigned value = 0);
         ~semaphore ();
 
         void wait (void);