Changes

gitignore: remove debian section

Having 'debian/libosmo-gprs-llc*' and similar in gitignore leads to
unexpected behavior when making releases:

* When bumping libversion, we need to rename related install files, e.g.:
  debian/libosmo-gprs-llc0.install -> debian/libosmo-gprs-llc1.install
* "git add -A" will not add the new files, instead they just show up as
  deleted
* this is easy to miss among the other changes, and it leads to a
  potentially confusing build error:
  dpkg-shlibdeps: error: cannot find library libosmo-gprs-llc.so.1 needed by debian/libosmo-gprs-gmm1/usr/lib/x86_64-linux-gnu/libosmo-gprs-gmm.so.1.0.0 (ELF format: 'elf64-x86-64' abi: '0201003e00000000'; RPATH: '')

Remove the entire debian dir from gitignore, it is not worth having it
in there.

Change-Id: I508242eef770c6fa467f5371587491147e9fc5eb