upnp: install to appropriate directory

Change-Id: I4c4e0161e9c256a9b95a7a8b9f2f786894dcce01

CMakeLists.txt

@@ -46,9 +46,9 @@ list (APPEND dhtnet_HEADERS
     include/fileutils.h
     include/string_utils.h
     include/ip_utils.h
-    include/mapping.h
-    include/upnp_context.h
-    include/upnp_control.h
+    include/upnp/mapping.h
+    include/upnp/upnp_context.h
+    include/upnp/upnp_control.h
 )
 
 add_library(dhtnet ${dhtnet_SOURCES})