stages/rpm.macros: new-line terminate values

Needed if we want to specify multiple values and even for the
single value use case this is the right thing to do.

stages/org.osbuild.rpm.macros

@@ -60,7 +60,7 @@ def main(tree, options):
     with open(path, "w", encoding="utf-8") as f:
         for k, v in options["macros"].items():
             value = make_value(k, v)
-            line = f"%{k} {value}"
+            line = f"%{k} {value}\n"
             f.write(line)