Tidy up the string utilities.

Change-Id: Ifd19a737b4a6a3e333aac64909a4694f7169786e
Reviewed-on: https://code-review.googlesource.com/6832
Reviewed-by: Paul Wankadia <junyer@google.com>

BUILD

diff --git a/BUILD b/BUILD
index 87a7c0c..1a8aecb 100644
--- a/BUILD
+++ b/BUILD
@@ -47,8 +47,8 @@
         "util/rune.cc",
         "util/sparse_array.h",
         "util/sparse_set.h",
-        "util/stringprintf.cc",
         "util/strutil.cc",
+        "util/strutil.h",
         "util/utf.h",
         "util/util.h",
     ],