package/mc: add optional support for libssh2

When libssh2 was compiled before, mc will use it as optional dependency:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/bin/mc | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libncurses.so.5]
 0x00000001 (NEEDED)                     Shared library: [libssh2.so.1]
 0x00000001 (NEEDED)                     Shared library: [libz.so.1]
 0x00000001 (NEEDED)                     Shared library: [libgcrypt.so.20]
 0x00000001 (NEEDED)                     Shared library: [libgpg-error.so.0]
 0x00000001 (NEEDED)                     Shared library: [libglib-2.0.so.0]
 0x00000001 (NEEDED)                     Shared library: [libpthread.so.1]
 0x00000001 (NEEDED)                     Shared library: [libintl.so.8]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]
 0x00000001 (NEEDED)                     Shared library: [libiconv.so.2]

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