Fix build of LoggingAgent's FilterSupport

[wfarr]

While compiling the apache2 module, I was seeing errors due to printf being undefined in the scope of FilterSupport.

This commit just includes stdio.h to ensure the printf is defined.

[FooBarWidget]

Thanks for the report, I've fixed this.

[jnewland]

At least 2 of our customers have had Passenger upgrades break because of this. Can you push a new gem?

[wfarr]

We've seen a handful of customers now break Passenger when upgrading to 3.0.5 due to the build failing.

We'd really like to see a 3.0.5.1 rolled out with the build fixes ASAP if possible.