MDEV-12096: Fix ln syntax in debian/rules for libmariadb3 symlink creation

Compatibility links ended up looking like this:
libmysqlclient.so.18 -> /tmp/buildd/mariadb-10.2-10.2.0/debian/tmp/usr/lib/i386-linux-gnu/libmariadb.so.3

This change fixes ln syntax to create links with correct target paths.

debian/rules

@@ -162,12 +162,12 @@ 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 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadbclient.so.18
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadbclient.so.18
 
 	# Install libmysqlclientclientXX compatibility links
-	ln -s $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.18
-	ln -s $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.19
-	ln -s $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.20
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.18
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.19
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmysqlclient.so.20
 
 	touch $@