build CHANGE remove plugins_exts.h from public headers

Extension's API is not yet ready, so remove it for now, it will be added
later.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5e7491b..e450f23 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -154,7 +154,6 @@
     src/in.h
     src/parser_data.h
     src/parser_schema.h
-    src/plugins_exts.h
     src/plugins_types.h
     src/out.h
     src/printer_data.h