With -Wformat-overflow gcc warns about calls to formatted input/output
function "sprintf" that might overflow the destination buffer.

In this case gcc does not know the upper bound of tm_min and assumes
that up to 11 bytes might be written to buffer (3 bytes). But we know
that tm_min can only be within the range 0 to 59.

mod_say_ja.c: In function 'ja_say_time':
mod_say_ja.c:376:35: error: '%d' directive writing between 2 and 10 bytes into a region of size 3 [-Werror=format-overflow=]
                  sprintf(buffer, "%d", tm.tm_min);
                                   ^~
mod_say_ja.c:376:34: note: directive argument in the range [11, 2147483647]
                  sprintf(buffer, "%d", tm.tm_min);
                                  ^~~~
mod_say_ja.c:376:18: note: 'sprintf' output between 3 and 11 bytes into a destination of size 3
                  sprintf(buffer, "%d", tm.tm_min);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This commits adds a hint for gcc, which silences the warning.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>