Add missing defines for STB

Author: musshorn

src/example1.cpp

@@ -38,6 +38,9 @@
 #include <nanogui/renderpass.h>
 #include <iostream>
 #include <memory>
+
+#define STB_IMAGE_STATIC
+#define STB_IMAGE_IMPLEMENTATION
 #include <stb_image.h>
 
 using namespace nanogui;