examples: add import prebuilt example

examples/build_system/cmake/import_prebuilt/main/main.c

@@ -0,0 +1,17 @@
+/* Import Prebuilt Library Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+// Include the prebuilt library's header file so as to be able 
+// to reference `prebuilt_func` here.
+#include "prebuilt.h"
+
+void app_main(void)
+{
+    prebuilt_func();
+}