configure, mysql plugin: Check for mysql.h as well.

Up to now, only mysql/mysql.h has been checked for. However, mysql_config
--cflags usually adds the complete path to mysql.h to the include flags. In
most setups, mysql/mysql.h can be found in the search path as well (usually
the header is available in something like /usr/include/mysql/mysql.h) so this
issue has not been found so far. However, if that's not the case, the build
will fail.

Thanks to Dusty Doris <collectd@dusty.name> for reporting this.