[1870] make sure install some more headers under dns/ and util/

there may be more, but at least this makes (b10-)host compile as a 3rd party
app.

src/lib/dns/Makefile.am

@@ -149,6 +149,7 @@ libdns___include_HEADERS = \
 	messagerenderer.h \
 	name.h \
 	question.h \
+	opcode.h \
 	rcode.h \
 	rdata.h \
 	rrparamregistry.h \

src/lib/util/Makefile.am

@@ -33,3 +33,6 @@ EXTRA_DIST = python/pycppwrapper_util.h
 
 libb10_util_la_LIBADD = $(top_builddir)/src/lib/exceptions/libb10-exceptions.la
 CLEANFILES = *.gcno *.gcda
+
+libb10_util_includedir = $(includedir)/$(PACKAGE_NAME)/util
+libb10_util_include_HEADERS = buffer.h