nmap: fix libssh2 build failure

nmap: fix libssh2 build failure

This commit adds a patch that fixes the libssh2 AC_CHECK_LIB() check,
and makes sure it is taken into account by doing an autoconf in the
package. We can't autoreconf as the package doesn't autoreconf
properly, so we do a manual autoconf.

Based on prior work from Baruch Siach and Max Filippov.

Fixes:

  http://autobuild.buildroot.net/results/9e636919c98cd31b5067c8306d0e481a672434cf
  http://autobuild.buildroot.net/results/912561f505ad10d1eaa96dbe247d5838e9968e14

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>