folly/AtomicHashMap-inl.h: avoid -Wsign-compare warning/error (easy)

folly/AtomicHashMap-inl.h: avoid -Wsign-compare warning/error (easy)

Summary:
* folly/AtomicHashMap-inl.h (insertInternal): Do not declare
nextMapIdx to be signed ("int"), since we compare it against
variables of unsigned type.  Otherwise, gcc-4.9 would give this
diagnostic:
folly/AtomicHashMap-inl.h:99:18: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
complaining about this comparison:
if (nextMapIdx >= kNumSubMaps_ ||

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

Reviewed By: philipp@fb.com

Subscribers: folly-diffs@

FB internal diff: D1770683

Tasks: 5941250

Signature: t1:1770683:1420683505:4452c7cb4f14f098be089601be906884036348ed