folly/test/ForeachTest.cpp: avoid -Wsign-compare error

Summary:
* folly/test/ForeachTest.cpp: Change a "1" to "1U", so it matches
the signedness of the size_t upper bound.
Otherwise, gcc-4.9 fails with e.g.,
folly/Foreach.h:194:16: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

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: andrei.alexandrescu@fb.com

Subscribers: folly-diffs@

FB internal diff: D1770603

Tasks: 5941250

Signature: t1:1770603:1420679246:56ef62ac7fa4413a4ad6310c3381a12bdc59e64c

folly/test/ForeachTest.cpp

@@ -266,7 +266,7 @@ BENCHMARK(ForEachDescendingManual, iters) {
 
 BENCHMARK(ForEachRangeR, iters) {
   int sum = 1;
-  FOR_EACH_RANGE_R (i, 1, iters) {
+  FOR_EACH_RANGE_R (i, 1U, iters) {
     sum *= i;
   }
   doNotOptimizeAway(sum);