Cycles: Use explicit qualifier for single-argument constructors

Almost in all cases we want such constructors to be explicit, there are
exceptions but only in few places.

intern/cycles/render/attribute.h

@@ -134,8 +134,8 @@ public:
 	AttributeElement triangle_element, curve_element;
 	int triangle_offset, curve_offset;
 
-	AttributeRequest(ustring name_);
-	AttributeRequest(AttributeStandard std);
+	explicit AttributeRequest(ustring name_);
+	explicit AttributeRequest(AttributeStandard std);
 };
 
 /* AttributeRequestSet