Fix building angleutils.cpp on Linux

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/221053
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>

AUTHORS

@@ -22,6 +22,7 @@ Mozilla Corporation
 Turbulenz
 Klarälvdalens Datakonsult AB
 Microsoft Open Technologies, Inc.
+NVIDIA Corporation
 
 Jacek Caban
 Mark Callow

CONTRIBUTORS

@@ -86,3 +86,6 @@ Jacek Caban
 Microsoft Open Technologies, Inc.
 Cooper Partin
 Austin Kinross
+
+NVIDIA Corporation
+ Olli Etuaho

src/common/angleutils.cpp

@@ -6,6 +6,7 @@
 
 #include "common/angleutils.h"
 
+#include <stdio.h>
 #include <vector>
 
 std::string FormatString(const char *fmt, va_list vararg)