Add a few missing headers

Fixes build with gcc 4.3.2 on Debian.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>
diff --git a/emulator/qtools/dmtrace.cpp b/emulator/qtools/dmtrace.cpp
index a27193a..3efc10a 100644
--- a/emulator/qtools/dmtrace.cpp
+++ b/emulator/qtools/dmtrace.cpp
@@ -4,6 +4,7 @@
 #include <stdlib.h>
 #include <inttypes.h>
 #include <string.h>
+#include <unistd.h>
 #include "dmtrace.h"
 
 static const short kVersion = 2;