README: Reflect missing --getopt in configure

83760814 ("configure: Check for new-enough getopt") has replaced
proposed patch and doesn't introduce --getopt option in configure.
Instead, `configure` and `run_tests.sh` expect proper getopt to be
available in PATH.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200924100613.71136-1-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>