Fix compile with gcc47
Bug #419635
https://bugs.gentoo.org/show_bug.cgi?id=419635
Submitted by Nikos Chantziaras

--- a/src/tools/depbuild.cpp
+++ b/src/tools/depbuild.cpp
@@ -25,6 +25,7 @@
 #include <string>
 #include <cstdio>
 #include <cstdlib>
+#include <unistd.h>
 using namespace std;
 
 #include "fileutil.h"