buildroot

sngrep: gnutls support also needs libgcrypt

The gnutls support in sngrep not only needs gnutls, but also
libgcrypt. So only enable gnutls support when both packages are
available.

Fixes:

  http://autobuild.buildroot.net/results/18fbb5748a6ad66ac6ec8141c3296d2b13fa40ac/

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