Fix rust-exp handling in makefile

Fix rust-exp handling in makefile

I noticed that the rust-exp handling in the Makefile differed from
that of other .y files.  I believe I noticed this by seeing a stray
"rm" in the build log.

This patch changes the Makefile to bring the rust-exp handling in line
with that of other .y files.

2016-06-10  Tom Tromey  <tom@tromey.com>

	* Makefile.in (COMMON_OBS): Remove rust-exp.o.
	(YYFILES): Add rust-exp.c.
	(YYOBJ): Add rust-exp.o.
	(local-maintainer-clean): Remove rust-exp.c.