debian/spec: Add build depedency to autoconf-archive

Change-Id: I56ff1c97ef7f11ac35eb78558cc7a0049f39cd56
diff --git a/contrib/osmo-smlc.spec.in b/contrib/osmo-smlc.spec.in
index ed2499e..c11bd88 100644
--- a/contrib/osmo-smlc.spec.in
+++ b/contrib/osmo-smlc.spec.in
@@ -24,6 +24,7 @@
 Group:          Hardware/Mobile
 URL:            https://osmocom.org/projects/osmo-smlc
 Source:         %{name}-%{version}.tar.xz
+BuildRequires:  autoconf
 BuildRequires:  automake >= 1.9
 BuildRequires:  libtool >= 2
 BuildRequires:  pkgconfig >= 0.20