debian: build now depends on libgnutls

In Change-Id Ic77866ce65acf524b768882c751a4f9c0635740b we introduced a
gnutls fall-back for random number generation, and made this a default
unless explicitly disabled at compile time.  This means the debian
package needs related build dependency.

Change-Id: I918e4b7bf1cb621679dce6339b3c4b69d653e2a6
Related: OS#1694
diff --git a/debian/control b/debian/control
index 8ba5583..64c4870 100644
--- a/debian/control
+++ b/debian/control
@@ -10,6 +10,7 @@
                dh-autoreconf,
                libdpkg-perl,
                git,
+               libgnutls28-dev,
                doxygen,
                libpcsclite-dev,
                pkg-config,