Add .spec file to source distribution

Makefile

@@ -407,7 +407,7 @@ SRC_DIST_DIRS += docs distrib $(filter-out docs distrib,$(SUBDIRS))
 SRC_DIST_FILES += \
 	configure.ac config.guess config.sub configure \
 	aclocal.m4 README ANNOUNCE HACKING LICENSE Makefile install-sh \
-	mk/bootstrap.mk \
+	ghc.spec.in mk/bootstrap.mk \
 	mk/boilerplate.mk mk/config.h.in mk/config.mk.in mk/opts.mk \
 	mk/paths.mk mk/package.mk mk/suffix.mk mk/target.mk \
 	mk/fptools.css mk/fix_install_names.sh