plugin types FEATURE add time-period plugin

Implemented plugin for the libnetconf2-netconf-server YANG module's
time-period type.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a838daa..b15c1f4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -106,7 +106,8 @@
     src/plugins_types/date_and_time.c
     src/plugins_types/hex_string.c
     src/plugins_types/xpath1.0.c
-    src/plugins_types/node_instanceid.c)
+    src/plugins_types/node_instanceid.c
+    src/plugins_types/time_period.c)
 
 set(libsrc
     src/ly_common.c