Expanded non-standard W_EXITCODE macro for Musl compatibility

diff -Naur vte-0.54.2.orig/src/widget.cc vte-0.54.2/src/widget.cc
--- vte-0.54.2.orig/src/widget.cc	2018-09-28 10:06:12.000000000 +0200
+++ vte-0.54.2/src/widget.cc	2018-11-06 15:04:30.275949689 +0100
@@ -20,8 +20,6 @@
 
 #include "widget.hh"
 
-#include <sys/wait.h> // for W_EXITCODE
-
 #include <new>
 #include <string>
 
@@ -126,7 +124,7 @@
 Widget::dispose() noexcept
 {
         if (m_terminal->terminate_child()) {
-                int status = W_EXITCODE(0, SIGKILL);
+                int status = (0) << 8 | (SIGKILL); // W_EXITCODE(ret, sig)
                 emit_child_exited(status);
         }
 }