Ensure header files using __BEGIN_DECLS include sys/cdefs.h.

We keep fixing these one-by-one; let's fix them all at once.

Found thus:

  find . -name *.h | xargs grep -L sys/cdefs.h | xargs grep -l BEGIN_DECL | xargs grep -L sys/types

Change-Id: I188842aa2484dc6176e96556d57c38a0f785b59b
diff --git a/libc/private/bionic_futex.h b/libc/private/bionic_futex.h
index 6c7fdbe..69658b1 100644
--- a/libc/private/bionic_futex.h
+++ b/libc/private/bionic_futex.h
@@ -29,6 +29,7 @@
 #define _BIONIC_FUTEX_H
 
 #include <linux/futex.h>
+#include <sys/cdefs.h>
 
 __BEGIN_DECLS