Update changelog for material.h include hittable.h

Author: hollasch

CHANGELOG.md

@@ -10,6 +10,7 @@ Change Log / Ray Tracing in One Weekend
 ### Common
   - Fix    -- Big improvement to print version listing font size (#1595) and more compact line
               height for code listings in both print and browser.
+  - Change -- Include hittable.h from material.h; drop `hit_record` forward declaration (#1609)
 
 ### In One Weekend
   - Fix    -- Fixed usage of the term "unit cube" for a cube of diameter two (#1555, #1603)