doc: Add gmtime_r() to rule A.5 exceptions

According to commit 400c1fa7 which
introduced rule A.5 to the coding guidelines, the initial exception list
was based on the non-standard function availability in minimal libc.

gmtime_r() however, has not been included to the list, although it's
present in minimal libc since 3e8df8b3.

This commit fixes this, by adding gmtime_r() to the rule A.5 exception
list.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>