pw_string: riscv-gcc toolchain round is not in std

Change-Id: Id66de2aad106178c2fe17f261d99f787fff4ce6f

pw_string/type_to_string.cc

diff --git a/pw_string/type_to_string.cc b/pw_string/type_to_string.cc
index 6d2769e..3475b12 100644
--- a/pw_string/type_to_string.cc
+++ b/pw_string/type_to_string.cc
@@ -153,7 +153,7 @@
   if (std::isfinite(value) &&
       std::abs(value) <
           static_cast<float>(std::numeric_limits<int64_t>::max())) {
-    return IntToString<int64_t>(std::round(value), buffer);
+    return IntToString<int64_t>(::round(value), buffer);
   }
 
   // Otherwise, print inf or NaN, if they fit.