This fixes the libstdc++ build after commit 4b6b8cf6 ("[cleanup] Start
cleaning up utils.h"), as string-util.cc was including <cmath> indirectly
via src/conversions.h.

Bug: chromium:819294
Change-Id: I5be7eac355d0c316a432eab4c50ae66d885b03d7
Reviewed-on: https://chromium-review.googlesource.com/c/1491591Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59905}