cmake: Add missing inclue dir.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index efe6b8b..b6cfb89 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -49,6 +49,7 @@
 
 # check subdirectories
 add_subdirectory(doc)
+add_subdirectory(include)
 add_subdirectory(src)
 if (UNIT_TESTING)
     add_subdirectory(example)