vsnprintf is defined in stdio.h according to the standard. It was not in
the include chain of angleutils.cpp, which broke the build on Linux on
some configurations. Seems like other toolchains include the function in
some non-standard way.

BUG=angle:761

Change-Id: Idcd75776e8a9b83dad182d1b4bac0beac006c6ac
Reviewed-on: https://chromium-review.googlesource.com/221053Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>