remove autogenerated file libs/ldns/install-sh

Re-bootstrapping is needed after this change.  Previously CentOS 5
systems wouldn't build correctly without this file in tree, but we've
adjusted bootstrap to compensate.

libs/.gitignore

@@ -887,7 +887,6 @@ missing
 !/ldns/drill/install-sh
 !/ldns/examples/config.h.in
 !/ldns/examples/configure
-!/ldns/install-sh
 !/ldns/ldns/config.h.in
 !/libdingaling/config.guess
 !/libdingaling/config.sub