Add a missing file 48-spacing.conf

Fixes https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/310
diff --git a/conf.d/Makefile.am b/conf.d/Makefile.am
index 502ac87..8474722 100644
--- a/conf.d/Makefile.am
+++ b/conf.d/Makefile.am
@@ -82,6 +82,7 @@
 	40-nonlatin.conf		\
 	45-generic.conf			\
 	45-latin.conf			\
+	48-spacing.conf			\
 	49-sansserif.conf		\
 	50-user.conf			\
 	51-local.conf			\