Fix #line directive path in ast.c template

The #line directive in templates/src/ast.c.erb referenced
"prism/templates/src/..." which is unrelated to this project.
Use "templates/src/..." instead so the generated src/ast.c points
to the correct source path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: soutaro