wireguard: selftests: set RISCV_ISA_FALLBACK on riscv{32,64}

This option is needed to continue booting with QEMU. Recent changes that
made this optional meant that it gets unset in the test harness, and so
WireGuard CI has been broken. Fix this by simply setting this option.

Cc: stable@vger.kernel.org
Fixes: 496ea826

 ("RISC-V: provide Kconfig & commandline options to control parsing "riscv,isa"")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>