mysql: fix include path reported by mysql_config

mysql_config should point to the directory where the MySQL include files
can be found and not its parent.

This fixes building modphp with mysql enabled, and allows cherokee to
actually enable mysql support.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>