keys.c: fix build with libressl >= 3.8.1

keys.c: fix build with libressl >= 3.8.1

libressl >= 3.8.1 removed ENGINE support:
"ENGINE support was removed and OPENSSL_NO_ENGINE is set. In spite
of this, some stub functions are provided to avoid patching some
applications that do not honor OPENSSL_NO_ENGINE."
resulting in the following build failure with gost:

./keys.c:167:35: error: 'ENGINE_METHOD_ALL' undeclared (first use in this function)
  167 |         if(!ENGINE_set_default(e, ENGINE_METHOD_ALL)) {
      |                                   ^~~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/37cc05b78a7004caa1b45d896121f059a4f8ca00

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>