Examples: Fix OpenGL3 example Makefile

examples/opengl3_example/Makefile

@@ -20,7 +20,7 @@ UNAME_S := $(shell uname -s)
 
 ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
-	CXXFLAGS = -I../../ `pkg-config --cflags glfw3`
+	CXXFLAGS = -I../../ -I../opengl_example/gl3w `pkg-config --cflags glfw3`
 	CXXFLAGS += -Wall
 	LIBS = `pkg-config --static --libs glfw3`
 endif