[plt-scheme] 'make clean' doesn't clean up libmzscheme3m.a

* src/mzscheme/Makefile.in (clean at NOT_OSX@): Delete
libmzscheme3m. at LIBSFX@, too.

Index: Makefile.in
===================================================================
--- Makefile.in	(revision 237)
+++ Makefile.in	(working copy)
@@ -223,7 +223,7 @@
 	/bin/rm -rf tmp[123456789] tests/tmp[123456789] tests/sub[123]
 	/bin/rm -f tests/*~
 	/bin/rm -f *. at LTO@ core
-	/bin/rm -f mzdyn.o libmzgc. at LIBSFX@ libmzscheme. at LIBSFX@ libdl.a mzscheme mzscheme.multiboot
+	/bin/rm -f mzdyn.o libmzgc. at LIBSFX@ libmzscheme. at LIBSFX@ libmzscheme3m. at LIBSFX@ libdl.a mzscheme mzscheme.multiboot
 	/bin/rm -f include/macosxpre
 	/bin/rm -f include/macosxpre.p
 	cd gc; $(MAKE) clean