Add #include <cmath> for std::ceil() #113

A couple files that use the std::ceil() math ceiling function were not
including the required header <cmath> and this seemed to have been
causing build errors on some Linux distributions.

Add the necessary #include <cmath> statement to both files.

src/wx/macsupport.mm

@@ -1,3 +1,4 @@
+#include <cmath>
 #import <Foundation/Foundation.h>
 #import <Cocoa/Cocoa.h>
 

src/wx/panel.cpp

@@ -1,3 +1,4 @@
+#include <cmath>
 #include <wx/dcbuffer.h>
 #include <SDL_joystick.h>