filemq doesn't build with gcc 7.1 due to Wformat-truncation

[RomainNaour]

Hi,

From: src/fmq_server_engine.inc:
//  Set log file prefix; this string will be added to log data, to make
//  log data more searchable. The string is truncated to ~20 chars.

The code intend to truncate the string to ~20 chars but this behaviour
throw an error with new compiler (gcc >= 7.1).

See:
http://autobuild.buildroot.net/results/434/43452430edf44cbcef6ea3a5ab80ac0ff406208f

This can be avoided by disabling this warning with -Wno-format-truncation.

Best regards,
Romain