Use operator() instead of apply() in endgames

It is more idiomatic for a functor (a function object) as are
the endgames.

Suggested by Rein Halbersma.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/material.h

@@ -97,12 +97,12 @@ inline ScaleFactor MaterialInfo::scale_factor(const Position& pos, Color c) cons
   if (!scalingFunction[c])
       return ScaleFactor(factor[c]);
 
-  ScaleFactor sf = scalingFunction[c]->apply(pos);
+  ScaleFactor sf = (*scalingFunction[c])(pos);
   return sf == SCALE_FACTOR_NONE ? ScaleFactor(factor[c]) : sf;
 }
 
 inline Value MaterialInfo::evaluate(const Position& pos) const {
-  return evaluationFunction->apply(pos);
+  return (*evaluationFunction)(pos);
 }
 
 inline Score MaterialInfo::material_value() const {