[mod_sofia] Pass api_on_ prefix to switch_channel_api_on in sofia_glue_set_extra_headers

[jacky-w-j-li]

sofia_glue_set_extra_headers() built a single execute_on_<prefix>prefix variable name and passed it to both switch_channel_execute_on() and switch_channel_api_on().

Both functions match channel variables by prefix (strncasecmp(var, variable_prefix, strlen(variable_prefix)) in src/switch_channel.c), so switch_channel_api_on() also matched the execute_on_* variables and ran their values a second time as API commands. API commands have no session context, so scripts invoked this way saw a NULL session, and any logic in an execute_on_sip_ph_prefix / execute_on_sip_rh_prefix (and other SIP header prefix) hook ran twice per SIP message.

Build the api_on_<prefix>prefix variable name before calling switch_channel_api_on(), matching the pattern already used for execute_on_sip_extra_headers / api_on_sip_extra_headers just below.

Resolves: #3005