Add <libc-diag.h> to simplify backports.

Added upstream as part of this commit:

commit e15f7de60c26bb75fe1923b17c5f0461164d1a41
Author: Zack Weinberg <zackw@panix.com>
Date:   Sun Nov 20 20:46:30 2016 -0500

    Split DIAG_* macros to new header libc-diag.h.

diff --git a/include/libc-diag.h b/include/libc-diag.h
new file mode 100644
index 0000000000000000..5876a41e8e5fcc15
--- /dev/null
+++ b/include/libc-diag.h
@@ -0,0 +1 @@
+#include <libc-internal.h>