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

libc/netbsd/net/reentrant.h

@@ -102,6 +102,7 @@
 
 #include <pthread.h>
 #include <signal.h>
+#include <sys/cdefs.h>
 
 #define	mutex_t			pthread_mutex_t
 #define	MUTEX_INITIALIZER	PTHREAD_MUTEX_INITIALIZER