Remove static initializer from BigIntegerUtils.cc

which was being caused by including <iostream> instead of <ostream>.
Since GN uses source_sets instead of static_libraries by default,
the static initializer was making its way into Chrome, which was bad.

R=tsepez@chromium.org
BUG=559766

Review URL: https://codereview.chromium.org/1510353003 .
diff --git a/third_party/bigint/BigIntegerUtils.hh b/third_party/bigint/BigIntegerUtils.hh
index d2f81f4..999cdd2 100644
--- a/third_party/bigint/BigIntegerUtils.hh
+++ b/third_party/bigint/BigIntegerUtils.hh
@@ -9,7 +9,7 @@
 
 #include "BigInteger.hh"
 #include <string>
-#include <iostream>
+#include <ostream>
 
 /* This file provides:
  * - Convenient std::string <-> BigUnsigned/BigInteger conversion routines