[pjsip] allow_contact_rewrite is pj_bool_t, but uses 2 as a value to be disabled

[Alexei Kuznetsov]

Hi,

In pjsua_acc.c we discovered that allow_contact_rewrite must be 2  
(line 652) in order to disable that feature. However, in pjsua.h it's  
declared as bool.

Alexei