ast.h: Add `extern` for `ast_expr_tombstone`.
ast.h: Add `extern` for `ast_expr_tombstone`.

This was missing the `extern` declaration, so multiple compilation
units including `libre/ast.h` by way of `src/libre/parser.act` were
defining the symbol.

Reported in #115 (tangential to the main issue).

katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm