vm.c, codedump.c: undef CASE macro

Add #undef CASE at end of files to prevent macro redefinition
warnings in amalgamation builds.

Co-authored-by: Claude <noreply@anthropic.com>

src/codedump.c

@@ -648,3 +648,4 @@ mrb_codedump_all(mrb_state *mrb, struct RProc *proc)
   mrb_codedump_all_file(mrb, proc, stdout);
 #endif
 }
+#undef CASE