formula: add missing SPDX require.

https://github.com/Homebrew/formulae.brew.sh/pull/328/checks?check_run_id=1102740199#step:6:5

Library/Homebrew/formula.rb

@@ -23,6 +23,7 @@ require "language/python"
 require "tab"
 require "mktemp"
 require "find"
+require "utils/spdx"
 
 # A formula provides instructions and metadata for Homebrew to install a piece
 # of software. Every Homebrew formula is a {Formula}.