d/control: libosmocore-dev: depend on libmnl-dev

Let libosmocore-dev depend on libmnl-dev, as we build the debian package
without --disable-libmnl and so libosmocore.pc lists libmnl in requires.
This patch should fix the following error when trying to build any
debien package depending on libosmocore-dev:

  Package 'libmnl', required by 'libosmocore', not found

A similar change in the rpm spec is not be needed, as rpm adds depends
from pc files automatically.

Fixes: 4eb89afa ("configure --enable-libmnl: Add libmnl to libosmocore.pc.in Requires")
Change-Id: I7fd578fcc22c0ea6ac04bdc26dc1741e4e26767c
diff --git a/debian/control b/debian/control
index 2b8504a..bbf9244 100644
--- a/debian/control
+++ b/debian/control
@@ -357,6 +357,7 @@
          libtalloc-dev (>= 2.1.0),
          libsctp-dev,
          libusb-1.0-0-dev,
+         libmnl-dev,
          ${misc:Depends}
 Description: Development headers for Open Source MObile COMmunications CORE library
  The header files provided by this package may be used to develop