include/config.mk: add -Wold-style-definition

Make compiler warn about old-style prototypes which were commonly
used before ANSI C. Using old-style function definition could result
in bugs that are not tracked down at compile time.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>