[6.0] 7cf202583 promote vtree.h to vmod include

[Dridi Boukelmoune]

commit 7cf202583c4e012405766e945042a35af76f8089
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Tue Nov 6 11:47:11 2018 +0100

    promote vtree.h to vmod include
    
    Now that we include it in cache.h, it is required by vmods
    
    Ref #2813

diff --git a/include/Makefile.am b/include/Makefile.am
index 98050fd78..01755e09d 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -71,6 +71,7 @@ nobase_pkginclude_HEADERS += \
 	vsha256.h \
 	vtcp.h \
 	vtim.h \
+	vtree.h \
 	vrnd.h
 
 # Private headers
@@ -98,7 +99,6 @@ nobase_noinst_HEADERS = \
 	vsub.h \
 	vss.h \
 	vtcp.h \
-	vtree.h \
 	vus.h
 
 ## keep in sync with lib/libvcc/Makefile.am