[ruby/erb] Enable frozen_string_literal in all files

(https://github.com/ruby/erb/pull/49)

I was surprised to see erb show up when I was using memory_profiler on
my app. ERB::Compiler#compile has a blank string literal, and it
ended up allocating some 41532 blank strings for a relatively small surface
area.

https://github.com/ruby/erb/commit/b7e45c2bdc

lib/erb/compiler.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::Compiler
 #

lib/erb/def_method.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::DefMethod
 #

lib/erb/util.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 #--
 # ERB::Escape
 #