Fix missing include on some systems to use intptr_t. (#2036, #2054)

imgui_widgets.cpp

@@ -33,6 +33,11 @@ Index of this file:
 #include "imgui_internal.h"
 
 #include <ctype.h>      // toupper, isprint
+#if defined(_MSC_VER) && _MSC_VER <= 1500 // MSVC 2008 or earlier
+#include <stddef.h>     // intptr_t
+#else
+#include <stdint.h>     // intptr_t
+#endif
 
 // Visual Studio warnings
 #ifdef _MSC_VER