pw_varint: C support

This change exposes a subset of the pw::varint API for use in C code.

Change-Id: I7db2dd1d2622711785e23c8534de6119301c57c3
diff --git a/pw_varint/BUILD b/pw_varint/BUILD
index abfa158..78bc286 100644
--- a/pw_varint/BUILD
+++ b/pw_varint/BUILD
@@ -1,4 +1,4 @@
-# Copyright 2019 The Pigweed Authors
+# Copyright 2020 The Pigweed Authors
 #
 # Licensed under the Apache License, Version 2.0 (the "License"); you may not
 # use this file except in compliance with the License. You may obtain a copy of
@@ -38,6 +38,9 @@
 
 pw_cc_test(
     name = "varint_test",
-    srcs = ["varint_test.cc"],
+    srcs = [
+        "varint_test.c",
+        "varint_test.cc",
+    ],
     deps = ["//pw_varint"],
 )