Fix enabling logging in an OSGi environment

Currently, within an OSGi environment (such as apache karaf e.g.), when logging is requested through configuration, enabling it will fail inside LoggerFactory with "Logging cannot be activated, missing slf4j dependency" always.
This is due to the fact that the OSGi Metadata you already generate is incorrect/missing something.
This commit fixes this "Import-Package" statement by adding the org.slf4j package as an optional import package.

pom.xml

@@ -266,7 +266,7 @@
                             <Bundle-SymbolicName>org.mariadb.jdbc</Bundle-SymbolicName>
                             <Export-Package>org.mariadb.jdbc</Export-Package>
                             <Import-Package>
-                                javax.net;resolution:=optional,javax.net.ssl;resolution:=optional,javax.sql;resolution:=optional,javax.transaction.xa;resolution:=optional
+                                javax.net;resolution:=optional,javax.net.ssl;resolution:=optional,javax.sql;resolution:=optional,javax.transaction.xa;resolution:=optional,org.slf4j;resolution:=optional
                             </Import-Package>
                         </manifestEntries>
                     </archive>