Shaders: make constructors explicit by default.

src/Magnum/Shaders/DistanceFieldVector.h

@@ -93,7 +93,7 @@ mesh.draw(shader);
 */
 template<UnsignedInt dimensions> class MAGNUM_SHADERS_EXPORT DistanceFieldVector: public AbstractVector<dimensions> {
     public:
-        DistanceFieldVector();
+        explicit DistanceFieldVector();
 
         /**
          * @brief Set transformation and projection matrix

src/Magnum/Shaders/Vector.h

@@ -87,7 +87,7 @@ mesh.draw(shader);
 */
 template<UnsignedInt dimensions> class MAGNUM_SHADERS_EXPORT Vector: public AbstractVector<dimensions> {
     public:
-        Vector();
+        explicit Vector();
 
         /**
          * @brief Set transformation and projection matrix