Move OPENSSL_FALLTHROUGH to internal headers.

Having it in base.h pollutes the global namespace a bit and, in
particular, causes clang to give unhelpful suggestions in consuming
projects.

Change-Id: I6ca1a88bdd1701f0c49192a0df56ac0953c7067c
Reviewed-on: https://boringssl-review.googlesource.com/25464
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/decrepit/macros.h b/decrepit/macros.h
index c4ebeec..7888f0d 100644
--- a/decrepit/macros.h
+++ b/decrepit/macros.h
@@ -57,6 +57,8 @@
 #ifndef OPENSSL_HEADER_DECREPIT_MACROS_H
 #define OPENSSL_HEADER_DECREPIT_MACROS_H
 
+#include "../crypto/internal.h"
+
 
 // NOTE - c is not incremented as per n2l
 #define n2ln(c, l1, l2, n)                       \