bignum: no-op make_string_bn_inplace for LIBGCRYPT
bignum: no-op make_string_bn_inplace for LIBGCRYPT

Disable the 'make_string_bn_inplace' helper function for the LIBGCRYPT
build, rather than using '#error' to fail the build completely.

Signed-off-by: Jon Simons <44f878afe53efc66b76772bd845eb65944ed8232@jonsimons.org>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

nviennot/libssh,mwgoldsmith/ssh,taikoo/libssh,mwgoldsmith/libssh,DouglasHeriot/libssh,sebadoom/libssh,DouglasHeriot/libssh,mwgoldsmith/ssh,robxu9/libssh,wangshawn/libssh,mwgoldsmith/libssh,pouete/libssh,kedazo/libssh,taikoo/libssh,Distrotech/libssh,mwgoldsmith/ssh,kedazo/libssh,sebadoom/libssh,nviennot/libssh,sebadoom/libssh,kedazo/libssh,nviennot/libssh,pouete/libssh,pouete/libssh,Distrotech/libssh,mwgoldsmith/libssh,kedazo/libssh,wangshawn/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,nviennot/libssh,taikoo/libssh,wangshawn/libssh,robxu9/libssh,Distrotech/libssh,sebadoom/libssh,taikoo/libssh,wangshawn/libssh,pouete/libssh,Distrotech/libssh,robxu9/libssh,DouglasHeriot/libssh,robxu9/libssh,DouglasHeriot/libssh