Fix build against musl

Upstream-Status: Pending

diff --git a/src/widget.cc b/src/widget.cc
index 25e4b242243a..33bb06d68698 100644
--- a/src/widget.cc
+++ b/src/widget.cc
@@ -21,6 +21,9 @@
 #include "widget.hh"
 
 #include <sys/wait.h> // for W_EXITCODE
+#ifndef W_EXITCODE
+#define W_EXITCODE(ret, sig) ((ret) << 8 | (sig))
+#endif
 
 #include <exception>
 #include <new>