package/strongswan: fix broken dependency relation

package/strongswan: fix broken dependency relation

The AKA backend for 3GPP2 requires libgmp (see
https://wiki.strongswan.org/projects/strongswan/wiki/Autoconf). Since
the AKA backend for 3GPP2 is included by BR2_PACKAGE_STRONGSWAN_EAP,
when selecting a crypto backend different from
BR2_PACKAGE_STRONGSWAN_GMP, there is no guarantee the gmp package is
selected as well. When doing so, make fails since the package is in the
dependency chain but not selected:

  $ make
  Makefile:585: *** gmp is in the dependency chain of strongswan that has added it to its _DEPENDENCIES variable without selecting it or depending on it from Config.in.  Stop.
  make: *** [Makefile:23: _all] Error 2

To fix this, select BR2_PACKAGE_GMP when selecting BR2_PACKAGE_STRONGSWAN_EAP.

Signed-off-by: Martin Elshuber <martin.elshuber@theobroma-systems.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>