[build] inline color_unit::EMPTY in header

Related to #1603

Author: tstack

src/base/color_spaces.cc

@@ -294,9 +294,3 @@ lab_color::avg(const lab_color& other) const
         (this->lc_b + other.lc_b) / 2.0,
     };
 }
-
-namespace styling {
-
-constexpr color_unit color_unit::EMPTY = color_unit{transparent{}};
-
-}  // namespace styling

src/base/color_spaces.hh

@@ -203,6 +203,8 @@ private:
     }
 };
 
+constexpr color_unit color_unit::EMPTY{transparent{}};
+
 }  // namespace styling
 
 #endif