package/mutt: add optional support for libidn

package/mutt: add optional support for libidn

When libidn was compiled before, mutt will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/mutt | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libncurses.so.5]
 0x0000000000000001 (NEEDED)             Shared library: [libidn.so.11]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>