d/control: library binary packages are of priority optional

Michael Biebl says:
 <<
 please consider lowering the priority of the library packages to
 optional. I know lintian will complain about that not being policy
 compliant, but I think installing libraries because of their priority is
 a stupid idea (and discussion is currently ongoing to fix the debian
 policy in that regard).
 >>
Suggested-by: Michael Biebl <biebl@debian.org>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>

debian/control

@@ -52,6 +52,7 @@ Description: transitional dummy package
 
 Package: libxtables11
 Architecture: linux-any
+Priority: optional
 Section: libs
 Depends: ${misc:Depends}, ${shlibs:Depends}
 Replaces: iptables (<< 1.4.16.3-3)
@@ -73,6 +74,7 @@ Description: netfilter xtables library
  This package includes the development files of libxtables.
 
 Package: libiptc0
+Priority: optional
 Multi-Arch: same
 Architecture: linux-any
 Section: libs
@@ -106,6 +108,7 @@ Description: Development files for libiptc
  This package provides development files and static libraries.
 
 Package: libip4tc0
+Priority: optional
 Multi-Arch: same
 Architecture: linux-any
 Section: libs
@@ -136,6 +139,7 @@ Description: Development files for libiptc
  This package provides development files and static libraries.
 
 Package: libip6tc0
+Priority: optional
 Multi-Arch: same
 Architecture: linux-any
 Section: libs