add missing includes which break mac debug build

src/Envelope.h

@@ -12,6 +12,7 @@
 #define __AUDACITY_ENVELOPE__
 
 #include <stdlib.h>
+#include <algorithm>
 
 #include <wx/dynarray.h>
 #include <wx/brush.h>

src/TimeTrack.h

@@ -15,6 +15,8 @@
 #include <wx/brush.h>
 #include <wx/pen.h>
 
+#include <algorithm>
+
 class wxRect;
 class wxDC;
 class Envelope;