Use a source_set rather than a static_library

Some toolchains (e.g., the host toolchain on Mac OS X) cannot build this
code as a static_library because it does not contain any complication
units (and hence the .a would be empty). Instead, we can model this
library as a source_set, which both works and is more accurate.

Change-Id: Ib11e866b74297e1a7c8c5a51312d3945bf154788

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index d299d84..be5d05f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-static_library("json") {
+source_set("json") {
   sources = [
     "src/json.hpp"
   ]