Fix binary-dist target with xz/gzip

This was harmless but annoying: we forgot to take the compression
extention into account when copying the binary dist out of bindistprep
Signed-off-by: Austin Seipp <austin@well-typed.com>

Makefile

@@ -72,7 +72,7 @@ endif
 	$(MAKE) -r --no-print-directory -f ghc.mk phase=final $@
 
 binary-dist: binary-dist-prep
-	mv bindistprep/*.tar.bz2 .
+	mv bindistprep/*.tar.$(TAR_COMP_EXT) .
 
 binary-dist-prep:
 ifeq "$(mingw32_TARGET_OS)" "1"