Changed "std::vsnprintf" to "vsnprintf" to be able to build with the android-ndk.

src/string_util.cc

@@ -6,6 +6,8 @@
 #include <memory>
 #include <sstream>
 
+#include <stdio.h>
+
 #include "arraysize.h"
 
 namespace benchmark {
@@ -127,7 +129,7 @@ std::string StringPrintFImp(const char *msg, va_list args)
   // allocation guess what the size might be
   std::array<char, 256> local_buff;
   std::size_t size = local_buff.size();
-  auto ret = std::vsnprintf(local_buff.data(), size, msg, args_cp);
+  auto ret = vsnprintf(local_buff.data(), size, msg, args_cp);
 
   va_end(args_cp);
 
@@ -141,7 +143,7 @@ std::string StringPrintFImp(const char *msg, va_list args)
   // add 1 to size to account for null-byte in size cast to prevent overflow
   size = static_cast<std::size_t>(ret) + 1;
   auto buff_ptr = std::unique_ptr<char[]>(new char[size]);
-  ret = std::vsnprintf(buff_ptr.get(), size, msg, args);
+  ret = vsnprintf(buff_ptr.get(), size, msg, args);
   return std::string(buff_ptr.get());
 }