Change calling convention of ImGuiInputTextCallback

src/ImGui.NET/ImGuiTextEditCallback.cs

@@ -1,4 +1,7 @@
-namespace ImGuiNET
+using System.Runtime.InteropServices;
+
+namespace ImGuiNET
 {
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     public unsafe delegate int ImGuiInputTextCallback(ImGuiInputTextCallbackData* data);
 }