Added missing #includes.

Fixes compilation with GCC 4.7.0.

src/menu.cpp

@@ -25,6 +25,7 @@
 #include <algorithm>
 #include <math.h>
 #include <fstream>
+#include <unistd.h>
 
 #include "gmenu2x.h"
 #include "linkapp.h"

src/messagebox.cpp

@@ -23,6 +23,8 @@
 
 #include <SDL_gfxPrimitives.h>
 
+#include <unistd.h>
+
 using namespace std;
 
 MessageBox::MessageBox(GMenu2X *gmenu2x, const string &text, const string &icon) {

src/utilities.cpp

@@ -32,6 +32,7 @@
 #include <fstream>
 #include <iostream>
 #include <strings.h>
+#include <unistd.h>
 
 using namespace std;