bugfix: debian, fix *.so symlinks in libmariadb-dev

The symlink

/usr/lib/libmysqlclient.so -> /usr/lib/libmysqlclient.so.18

was invalid, because the library was not in /usr/lib.
The correct symlink is

/usr/lib/x86_64-linux-gnu/libmysqlclient.so -> /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18

debian/libmariadb-dev.install

 usr/bin/mysql_config
 usr/include/mysql
 usr/lib/*/libmariadb.so
+usr/lib/*/libmysqlclient.so
+usr/lib/*/libmariadbclient.so
 usr/lib/*/libmariadbclient.a
 usr/lib/*/libmysqlservices.a
 usr/share/aclocal/mysql.m4

debian/libmariadb-dev.links

-usr/lib/libmysqlclient.so.18	usr/lib/libmysqlclient.so
-usr/lib/libmysqlclient_r.so.18	usr/lib/libmysqlclient_r.so

debian/rules

@@ -162,6 +162,7 @@ override_dh_auto_install:
 	install -D -m 644 debian/mariadb-server-10.2.py $(TMP)/usr/share/apport/package-hooks/source_mariadb-10.2.py
 
 	# Install libmariadbclient18 compatibility links
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadbclient.so
 	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadbclient.so.18
 
 	# Install libmysqlclientclientXX compatibility links