Fixed compiler error about missing header file.

modsim/ass2/q5.c

@@ -20,7 +20,11 @@ double simpson(func_ptr f, double a, double b) {
 }
 
 double gauss(func_ptr f, double a, double b) {
-    return 0;
+    double s = (b - a) / sqrt(3),
+        // calculate abscissae
+        left  = (b + a - s) / 2.0,
+        right = (b + a + s) / 2.0;
+    return (b - a) * (f(left) + f(right)) / 2.0;
 }
 
 double integral(func_ptr f, method_ptr method, double a, double b) {