versioner: turn on -Wundef.

Poking around with -Wundef exposed a pretty horrifying bug, so we
should probably keep it turned on.

Bug: http://b/31496165
Change-Id: Iada279eec91ded0d59ff3841d5a3ce1a36e34c3a

tools/versioner/src/versioner.cpp

@@ -80,6 +80,7 @@ class HeaderCompilationDatabase : public CompilationDatabase {
     command.push_back("-Wall");
     command.push_back("-Wextra");
     command.push_back("-Werror");
+    command.push_back("-Wundef");
     command.push_back("-Wno-unused-macros");
     command.push_back("-Wno-unused-function");
     command.push_back("-Wno-unused-variable");