Move <sys/_sigdefs.h> and <sys/_errdefs.h> to private.

Bug: http://b/28067717
Change-Id: I02f78ad7293a15ff45e35e9c20f60d3d023931eb
diff --git a/libc/bionic/strerror_r.cpp b/libc/bionic/strerror_r.cpp
index d419fb1..a0a0809 100644
--- a/libc/bionic/strerror_r.cpp
+++ b/libc/bionic/strerror_r.cpp
@@ -31,7 +31,7 @@
 
 static const Pair _sys_error_strings[] = {
 #define  __BIONIC_ERRDEF(x,y,z)  { x, z },
-#include <sys/_errdefs.h>
+#include "private/bionic_errdefs.h"
   { 0, NULL }
 };
 
@@ -41,7 +41,7 @@
 
 static const Pair _sys_signal_strings[] = {
 #define  __BIONIC_SIGDEF(signal_number, signal_description)  { signal_number, signal_description },
-#include <sys/_sigdefs.h>
+#include "private/bionic_sigdefs.h"
   { 0, NULL }
 };