prctl.h: include sys/cdefs.h

prctl.h uses __BEGIN_DECLS but fails to include sys/cdefs.h
(where it's defined).  Code which includes prctl.h without
previously including sys/cdefs.h will fail to compile.

Fixed.

Change-Id: If4c9f3308f08b93596dcd00e351ae786807e9320
diff --git a/libc/include/sys/prctl.h b/libc/include/sys/prctl.h
index 359d684..00e5837 100644
--- a/libc/include/sys/prctl.h
+++ b/libc/include/sys/prctl.h
@@ -29,6 +29,7 @@
 #define _SYS_PRCTL_H
 
 #include <linux/prctl.h>
+#include <sys/cdefs.h>
 
 __BEGIN_DECLS