fixes build failure on ia64

It's a backport of upstream fix by Émeric Maschino
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=5a6c3296a7a90694ad4042f6256f3da6d4fa4ee8
https://bugs.gentoo.org/658014
--- a/gdb/nat/linux-ptrace.c	2017-01-21 14:48:42.000000000 +0100
+++ b/gdb/nat/linux-ptrace.c	2018-06-20 00:03:40.520317323 +0200
@@ -21,8 +21,6 @@
 #include "linux-procfs.h"
 #include "linux-waitpid.h"
 #include "buffer.h"
-#include "gdb_wait.h"
-#include "gdb_ptrace.h"
 #include <sys/procfs.h>
 
 /* Stores the ptrace options supported by the running kernel.
--- a/gdb/nat/linux-ptrace.h	2017-01-21 14:46:47.000000000 +0100
+++ b/gdb/nat/linux-ptrace.h	2018-06-20 00:04:28.706220951 +0200
@@ -21,6 +21,7 @@
 struct buffer;
 
 #include "nat/gdb_ptrace.h"
+#include "gdb_wait.h"
 
 #ifdef __UCLIBC__
 #if !(defined(__UCLIBC_HAS_MMU__) || defined(__ARCH_HAS_MMU__))