convbin: workaround gmtime_t for windows WIN32

[ourairquality]

which has gmtime_s.

[rtklibexplorer]

From AI code review:

gmtime_r() returns non-null on success, so the existing if (...) is correct. But Microsoft gmtime_s() returns 0 on success and nonzero on failure, so this branch only enters the block on failure and then uses tm anyway. Microsoft documents gmtime_s as “Zero if successful” and error code on failure.

Suggested fix:

#ifdef _MSC_VER
if (!gmtime_s(&tm, &st.st_mtime)) {
#else
if (gmtime_r(&st.st_mtime, &tm)) {
#endif

Is this a valid concern?