main: only allow --gdb option when compiled with gdb feature
main: only allow --gdb option when compiled with gdb feature

Without this cfg check, execution can reach an unreachable!() statement
in the arg handling code on non-gdb builds.

BUG=None
TEST=cargo build # without --features=gdb

Change-Id: I8bec7abbb39e658d5c8f45946fd4fc28f0ae97a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3668741
Reviewed-by: Anton Romanov <7c8c28e1283c1d87f2c3df4d6b5c74b6ae02bcd9@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm