libsoup: add option to enable HTTPS (SSL) support

By selecting BR2_PACKAGE_LIBSOUP_SSL the gnutls package is selected
and libsoup will be compiled with SSL support.

[Peter: pass --with-libgcrypt-prefix so configure finds it]
Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>