mjit.c: suppress unused-value warning

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62251 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -1062,7 +1062,7 @@ build_header_path(const char *basedir, size_t baselen, const char *dir, size_t d
     p = path;
     p = append_str2(p, basedir, baselen);
     p = append_str2(p, dir, dirlen);
-    append_str2(p, header_basename, sizeof(header_basename));
+    p = append_str2(p, header_basename, sizeof(header_basename));
     return path;
 }