Update range-sum-query-2d-mutable.cpp

Author: kamyu104

C++/range-sum-query-2d-mutable.cpp

@@ -186,7 +186,7 @@ class NumMatrix2 {
         }
     }
 
-    int lower_bit(int i) {
+    inline int lower_bit(int i) {
         return i & -i;
     }
 };