examples: add import prebuilt example

examples/build_system/cmake/import_prebuilt/prebuilt/main/CMakeLists.txt

@@ -0,0 +1,2 @@
+idf_component_register(SRCS "main.c"
+                    INCLUDE_DIRS "")

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

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+void app_main(void)
+{
+    printf("Hello World!\n");
+}