Don't use SWIG_exit() in C examples

There doesn't seem to be any reason for using it rather than just
returning from main() as usual, and it provokes warnings about
implicitly declared function when compiling them.

Examples/c/class/runme.c

@@ -39,6 +39,6 @@ int main(int argc, char **argv) {
   printf("%d shapes remain\n", Shape_nshapes_get());
   printf("Goodbye\n");
 
-  SWIG_exit(0);
+  return 0;
 }
 

Examples/c/exception/runme.c

@@ -42,6 +42,6 @@ int main() {
 
   Test_delete(t);
 
-  SWIG_exit(0);
+  return 0;
 }
 

Examples/c/simple/runme.c

@@ -10,6 +10,6 @@ int main(int argc, char **argv) {
   printf("Foo = %f\n", Foo);
   Foo = 3.1415926;
   printf("Foo = %f\n", Foo);
-  SWIG_exit(0);
+  return 0;
 }
 

Examples/c/std_vector/runme.c

@@ -38,5 +38,5 @@ int main() {
   
   Klass_delete(klass);
 
-  SWIG_exit(0);
+  return 0;
 }