Use gsmtap_inst_fd2() to remove compiler warning
gsmtap.c: In function 'osmo_st2_gsmtap_send_apdu':
gsmtap.c:69:9: warning: 'gsmtap_inst_fd' is deprecated: Use gsmtap_inst_fd2() instead [-Wdeprecated-declarations]
69 | rc = write(gsmtap_inst_fd(g_gti), buf, gross_len);
| ^~
Change-Id: I62de38175e63cb563635a01b025b805981e4b7a9
