Move FOLLY_DISABLE_ADDRESS_SANITIZER from Fiber::init()'s definition to declaration

Move FOLLY_DISABLE_ADDRESS_SANITIZER from Fiber::init()'s definition to declaration

Summary:
I just had topaggr built from trunk crash with https://phabricator.fb.com/P19825084. It seems that
placing `FOLLY_DISABLE_ADDRESS_SANITIZER` next to the function definition has no effect. Though
what's strange, https://phabricator.fb.com/D2036280 test plan says the false-positive crashes
stopped after that diff. Have we run the tao script on the latest revisions of that diff? Anyway,
clang/gcc docs clearly say the attributes belong to declarations, not definitions, so let's do that.

Test Plan:
Compiled, unit tests.

Reviewed By: aap@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant, soren

FB internal diff: D2042149

Signature: t1:2042149:1430681824:aea21b9a1fcddfd5585180ba7f526c3a6ee0c8bf