migration-exec: fix OpenBSD build warning

Add include directives to get WIFEXITED() and WEXITSTATUS() macros defined.

Fixes warnings:
  CC    migration-exec.o
/src/qemu/migration-exec.c: In function `exec_close':
/src/qemu/migration-exec.c:53: warning: implicit declaration of function `WIFEXITED'
/src/qemu/migration-exec.c:54: warning: implicit declaration of function `WEXITSTATUS'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

migration-exec.c

diff --git a/migration-exec.c b/migration-exec.c
index 93bde62..a8813b4 100644
--- a/migration-exec.c
+++ b/migration-exec.c
@@ -20,6 +20,8 @@
 #include "sysemu.h"
 #include "buffered_file.h"
 #include "block.h"
+#include <sys/types.h>
+#include <sys/wait.h>
 
 //#define DEBUG_MIGRATION_EXEC