folly/Format-inl.h: fix gcc-4.9-exposed shadowing warning/error (trivial)

folly/Format-inl.h: fix gcc-4.9-exposed shadowing warning/error (trivial)

Summary:
* folly/Format-inl.h (BaseFormatter): Move decls of P and END down
past the shadowed decls.  Otherwise, gcc-4.9 would emit this:
./folly/Format-inl.h:209:10: error: declaration of 'p' shadows a previous local [-Werror=shadow-local]
./folly/Format-inl.h:202:8: error: shadowed declaration is here [-Werror=shadow-local]
./folly/Format-inl.h:210:10: error: declaration of 'end' shadows a previous local [-Werror=shadow-local]
./folly/Format-inl.h:203:8: error: shadowed declaration is here [-Werror=shadow-local]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 tao/server && fbmake dbgo

Reviewed By: andrewjcg@fb.com

Subscribers: trunkagent, net-systems@, folly-diffs@

FB internal diff: D1766673

Tasks: 5941250

Signature: t1:1766673:1420569376:22c5c3a012715e479b27ba6f0bde1a3b079a3be3