Mark truffle nfi as a language provider too (#67)

Signed-off-by: Holger Hees <holger.hees@gmail.com>

Author: HolgerHees

org.graalvm.truffle.truffle-nfi/osgi.bnd

@@ -7,5 +7,7 @@ Import-Package: \
 Export-Package: \
   !NOTICE, \
   *;version=${project.version}
+Require-Capability: osgi.extender;filter:="(osgi.extender=osgi.serviceloader.registrar)"
+Provide-Capability: osgi.serviceloader;osgi.serviceloader="com.oracle.truffle.api.provider.TruffleLanguageProvider"
 -includeresource: \
   NOTICE