debian: Change python3 dependency to native arch

This small fix enables cross compilation using dpkg-buildpackage on multiarch:
https://wiki.debian.org/CrossCompiling#Building_with_dpkg-buildpackage

`apt build-dep -a<arch>` incorrectly installed non-native python for building

Change-Id: I85994447657cda757348855c3ee9978e8c7c2377

debian/control

@@ -17,7 +17,7 @@ Build-Depends: debhelper (>= 9),
                libtalloc-dev,
                libsctp-dev,
                libusb-1.0-0-dev,
-               python3
+               python3:native
 Standards-Version: 3.9.8
 Vcs-Git: git://git.osmocom.org/libosmocore.git
 Vcs-Browser: http://git.osmocom.org/libosmocore/