debian: depend on libsctp-dev

Since we merged the SGs interface, we include <netinet/sctp.h>, which
is provided by libsctp-dev.  This means that the Debian package should
depend on this.

It is expected that this will un-break the network:osmocom:nightly
builds.

Change-Id: I092e95ea970763c4008d3c7ff1b7028042574a64
diff --git a/debian/control b/debian/control
index e607d10..f6e8ace 100644
--- a/debian/control
+++ b/debian/control
@@ -10,6 +10,7 @@
                libtool,
                pkg-config,
                libdbi-dev,
+               libsctp-dev,
                libtalloc-dev,
                libsmpp34-dev (>= 1.12),
                libasn1c-dev (>= 0.9.28),