Test POSIX 1003.1 2016 chapter 13 "Headers".

And fix one thing that this found: apparently <stdlib.h> should also
make the various *WAIT* macros available.

Bug: N/A
Test: builds
Change-Id: Id879bf3c1bddd1170261a809e7280150a74d6b3d
diff --git a/tests/headers/Android.bp b/tests/headers/Android.bp
new file mode 100644
index 0000000..b44c296
--- /dev/null
+++ b/tests/headers/Android.bp
@@ -0,0 +1 @@
+subdirs = ["*"]