res_pjsip: Prevent "user=phone" being added multiple times to header

ast_sip_add_usereqphone adds "user=phone" to the header every time is is
called without checking whether the param already exists. Preventing
this by searching to string representation of header for "user=phone".

ASTERISK-26988 #close

Change-Id: Ib84383b07254de357dc6a98d91fc1d2c2c3719e6