mariadb: add dependency libevent

mariadb failed to build:
| make[2]: *** No rule to make target
  '/.../bitbake_build/tmp/sysroots/qemux86-64/usr/lib64/libevent.so',
  needed by 'tests/async_queries'.  Stop.
| make[2]: Leaving directory
  '/.../bitbake_build/tmp/work/core2-64-wrs-linux/mariadb/5.5.40-r0/build'
  CMakeFiles/Makefile2:6742: recipe for target 'tests/CMakeFiles/async_queries.dir/all' failed
| make[1]: *** [tests/CMakeFiles/async_queries.dir/all] Error 2

It is an rare case that when configure mariadb, libevent exists in
sysroot. But when do compile for mariadb, libevent is gone somehow. That
causes build fails. Run follow commands could reproduce this error:

$ bitbake libevent
$ bitbake mariadb -c cleansstate
$ bitbake mariadb -c configure
$ bitbake libevent -c clean
$ bitbake mariadb

Add if without libevent, test case tests/async_queries.c will not be built.
The case is helpful to do unit test. So add libevent as a dependency to fix the
error and make sure async_queries.c will be built.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>