util/string: Inline inner namespace

These functions are unique enough that we don't need a separate
namespace. We don't yet want to break existing callers so we need to
keep the inner namespace.

Change-Id: I1a0f1862a94dee0dbb0d3bb3ad241ba2302e2324
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/util/string.hpp b/include/stdplus/util/string.hpp
index 2f1558a..3c39df0 100644
--- a/include/stdplus/util/string.hpp
+++ b/include/stdplus/util/string.hpp
@@ -5,8 +5,6 @@
 
 namespace stdplus
 {
-namespace util
-{
 namespace detail
 {
 
@@ -40,6 +38,9 @@
 
 } // namespace detail
 
+inline namespace util
+{
+
 /** @brief Appends multiple strings to the end of the destination string
  *         in the most optimal way for the given inputs.
  *