debuginfod: Use gmtime_r instead of gmtime to avoid data race

debuginfod: Use gmtime_r instead of gmtime to avoid data race

Since we are multi-threaded using gmtime might cause a data race
because gmtime reuses a global struct to write data into. Make
sure that each thread uses their own struct tm and use gmtime_r
instead.

Signed-off-by: Mark Wielaard <mark@klomp.org>