Update libmariadb-dev.links to restore /usr/include/mysql compatibility...

Update libmariadb-dev.links to restore /usr/include/mysql compatibility symlinks lost when the default switched from 10.1 to 10.3 (Closes: #917266)

debian/libmariadb-dev.links

 usr/share/man/man1/mysql_config.1.gz usr/share/man/man1/mariadb_config.1.gz
+usr/include/mariadb/ usr/include/mysql
\ No newline at end of file