Makefile: Make libhtp available at install-rules stage

So far when "make install-rules" stage was executed, libhtp path was not
recognized as ldconfig does not run by this stage.
Set "LD_LIBRARY_PATH" since we already know the path where libhtp would
be.

Closes redmine ticket #2669.

Makefile.am

@@ -32,7 +32,7 @@ install-conf:
 
 install-rules:
 if HAVE_SURICATA_UPDATE
-	$(DESTDIR)$(bindir)/suricata-update \
+	LD_LIBRARY_PATH=$(libdir) $(DESTDIR)$(bindir)/suricata-update \
 		--suricata $(DESTDIR)$(bindir)/suricata \
 		--suricata-conf $(DESTDIR)$(sysconfdir)/suricata/suricata.yaml \
 		--no-test --no-reload