Deb: Recover include/mariadb wrongly removed in commit 9b10b418

The libmariadb-dev package must include this directory so that downstream
developers can intentionally include MariaDB libraries as they want.

This matches the contents of the official Debian libmariadb-dev package.

debian/libmariadb-dev.install

 usr/bin/mysql_config
 usr/include/mysql
+usr/include/mariadb
 usr/lib/*/libmariadb.so
 usr/lib/*/libmysqlclient.so
 usr/lib/*/libmariadbclient.so