fs/FileSystem: add TruncateFile()

src/fs/FileSystem.hxx

@@ -104,6 +104,13 @@ StatFile(Path file, struct stat &buf, bool follow_symlinks = true)
 
 #endif
 
+/**
+ * Truncate a file that exists already.  Throws std::system_error on
+ * error.
+ */
+void
+TruncateFile(Path path);
+
 /**
  * Wrapper for unlink() that uses #Path names.
  */