versioner: add support for preprocessing headers.

Bug: http://b/30170081
Change-Id: I9b662ca9e0fa3a1f9c0211594e851f5f9bef7266
diff --git a/tools/versioner/src/versioner.h b/tools/versioner/src/versioner.h
index 33d8084..986c25f 100644
--- a/tools/versioner/src/versioner.h
+++ b/tools/versioner/src/versioner.h
@@ -24,6 +24,13 @@
 
 extern bool verbose;
 
+#define D(...)             \
+  do {                     \
+    if (verbose) {         \
+      printf(__VA_ARGS__); \
+    }                      \
+  } while (0)
+
 static const std::unordered_map<std::string, std::set<Arch>> header_blacklist = {
   // Internal header.
   { "sys/_system_properties.h", supported_archs },