Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/cuckoocache.h

@@ -58,7 +58,7 @@ public:
      * @post All calls to bit_is_set (without subsequent bit_unset) will return
      * true.
      */
-    bit_packed_atomic_flags(uint32_t size)
+    explicit bit_packed_atomic_flags(uint32_t size)
     {
         // pad out the size if needed
         size = (size + 7) / 8;