fix: change to_openqasm cbit num

[beizhansl]

If we use the length of measures as the size of the cbit array, many problems will arise.
For example:

qc = QuantumCircuit(3)
qc.cx([0],[2])
qc.to_openqasm()

the qsam will be

include "qelib1.inc";
qreg q[4];
creg meas[1];
cx q[0],q[1];
measure q[0] -> meas[3];