preprocessor: increase max arguments of FOLLY_PP_FOR_EACH
preprocessor: increase max arguments of FOLLY_PP_FOR_EACH

Summary: EdenFS has a use-case for this macro but with 10+ arguments. Raise the maximum number of arguments from 7 to 15 to permit this.

Reviewed By: yfeldblum

Differential Revision: D26268587

fbshipit-source-id: be0b0579d854a93dc2983706a253370fe05905df

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly