Fix linker error in constant_expr testcase

Author: wsfulton

Examples/test-suite/constant_expr.i

@@ -21,6 +21,6 @@ isPointer = false
  * SWIG 3.0.4 until 4.1.0, `Unknown directive '%a'`.
  */
 int a;
-int test2(int b = 9%a);
+int test2(int b = 9%a) { return b; }
 
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