Fix build for folly_fb_platform

Summary:
This wasn't building with an error from a warning about
narrowing conversions---probably we didn't notice because it was
previously broken due to folly docs being broken forever.  Folly tests
should always pass.  (I want to add it to the fbcode-tests-must-pass
commit hook.)

Test Plan:
fbmake runtests_opt in this platform, except with stl_tests
turned off because it takes years to compile.

Reviewed By: andrewjcg@fb.com

FB internal diff: D792059

folly/experimental/io/test/AsyncIOTest.cpp

@@ -321,7 +321,7 @@ TEST(AsyncIO, ManyAsyncDataNotPollable) {
   {
     std::vector<TestSpec> v;
     for (int i = 0; i < 1000; i++) {
-      v.push_back({kAlign * i, kAlign});
+      v.push_back({off_t(kAlign * i), kAlign});
     }
     testReads(v, AsyncIO::NOT_POLLABLE);
   }
@@ -331,7 +331,7 @@ TEST(AsyncIO, ManyAsyncDataPollable) {
   {
     std::vector<TestSpec> v;
     for (int i = 0; i < 1000; i++) {
-      v.push_back({kAlign * i, kAlign});
+      v.push_back({off_t(kAlign * i), kAlign});
     }
     testReads(v, AsyncIO::POLLABLE);
   }