folly/Range.cpp: avoid -Werror=sign-compare error with gcc-4.9

folly/Range.cpp: avoid -Werror=sign-compare error with gcc-4.9

Summary:
* folly/Range.cpp (scanHaystackBlock): This method mistakenly
declared its "idx" (aka blockStartIdx) parameter to have signed type.
It is logically an unsigned type and is compared only with other
variables of unsigned type, so make it unsigned.
Here's the diagnostic:
folly/Range.cpp:202:44: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
folly/Range.cpp: In instantiation of 'size_t folly::detail::scanHaystackBlock(folly::StringPiece, folly::StringPiece, int64_t) [with bool HAYSTACK_ALIGNED = true; size_t = long unsigned int; folly::StringPiece = folly::Range<const char*>; int64_t = long int]':

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: robbert@fb.com, philipp@fb.com

Subscribers: folly-diffs@

FB internal diff: D1765590

Signature: t1:1765590:1420506036:7cbe2c454ad1f018a1c0aa5112a38bed1b2ac673