Add missing require in ImageMagick analyzer

ref: rails#45100 (comment)

Compare: https://github.com/rails/rails/blob/885734a85ef4f4578400bda72a7507dd09a85e64/activestorage/lib/active_storage/analyzer/image_analyzer/vips.rb#L5

Author: ghiculescu

activestorage/lib/active_storage/analyzer/image_analyzer/image_magick.rb

@@ -2,6 +2,7 @@
 
 begin
   gem "mini_magick"
+  require "mini_magick"
 rescue LoadError => error
   raise error unless error.message.include?("mini_magick")
 end