remove subdir include/internal on install

overseen this on an earlier commit

Signed-off-by: Peter S. Mazinger <ps.m@gmx.net>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Makefile.in

@@ -214,6 +214,7 @@ $(top_builddir)extra/scripts/unifdef: $(top_srcdir)extra/scripts/unifdef.c
 # a "y" here means the feature is enabled and so we should *not* rm it.
 # if the option expands to nothing though, we can punt the headers.
 HEADERS_RM- := \
+	internal \
 	dl-osinfo.h \
 	hp-timing.h \
 	_lfs_64.h \