Add ext/strscan/depend (#218)

`strscan` has no `ext/strscan/depend`, but ruby/ruby carries one. [Misc
#22206](https://bugs.ruby-lang.org/issues/22206) settled on each default
gem keeping its own dependency file, with `minimize_dependencies`
reducing it on the ruby/ruby side. Because `tool/sync_default_gems.rb`
clears the mapped directory before copying this repository over it,
every sync drops `ext/strscan/depend` from ruby/ruby and it has to be
restored by hand.

This adds the file exactly as `ruby extconf.rb --update-depend`
generates it. That path scans only this repository, so it names no
installed Ruby headers and no `$(hdrdir)` entries leak into the result,
which keeps the file usable on Rubies older than the one that generated
it. The content matches what ruby/ruby has today byte for byte.

Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt

ext/strscan/depend

@@ -0,0 +1,3 @@
+# AUTOGENERATED DEPENDENCIES START
+strscan.o: strscan.c
+# AUTOGENERATED DEPENDENCIES END