Revert "Exclude prism/generate-srcs.mk.rb from sync targets of prism"

This reverts commit 7ddc53b.

Author: nobu

tool/sync_default_gems.rb

@@ -330,7 +330,6 @@ def sync_default_gems(gem)
       rm_rf("test/prism/snapshots")
 
       rm("prism/extconf.rb")
-      `git checkout prism/generate-srcs.mk.rb`
     when "resolv"
       rm_rf(%w[lib/resolv.* ext/win32/resolv test/resolv ext/win32/lib/win32/resolv.rb])
       cp_r("#{upstream}/lib/resolv.rb", "lib")