change top_srcdir to top_builddir as the path_replacer.sh is a generated tool

problem was noticed by wlodek (and reproduced by me):

  make[4]: Entering directory
  `/home/wlodek/dev/ktest/src/bin/keactrl'

  /home/wlodek/dev/kea_test/tools/path_replacer.sh
  /home/wlodek/dev/kea_test/src/bin/keactrl/kea.conf.pre kea.conf

  /bin/bash: /home/wlodek/dev/kea_test/tools/path_replacer.sh: No
  such file or directory

src/bin/keactrl/Makefile.am

@@ -30,7 +30,7 @@ $(man_MANS):
 endif
 
 kea.conf: kea.conf.pre
-	$(top_srcdir)/tools/path_replacer.sh $(top_srcdir)/src/bin/keactrl/kea.conf.pre $@
+	$(top_builddir)/tools/path_replacer.sh $(top_srcdir)/src/bin/keactrl/kea.conf.pre $@
 
 if INSTALL_CONFIGURATIONS