debian: Split libosmocore into various libs and address lintian warnings

Package the various libraries separately. This will allow to install
libosmogsm3 and libosmogsm4 in parallel.

E: libosmocore-dev: non-empty-dependency_libs-in-la-file usr/lib/libosmocore.la
E: libosmocore-dev: non-empty-dependency_libs-in-la-file usr/lib/libosmogb.la
E: libosmocore-dev: non-empty-dependency_libs-in-la-file usr/lib/libosmogsm.la
E: libosmocore-dev: non-empty-dependency_libs-in-la-file usr/lib/libosmovty.la

W: libosmocore-dbg: wrong-bug-number-in-closes l54:#nnnn
W: libosmocore: wrong-bug-number-in-closes l54:#nnnn

W: libosmocore: package-name-doesnt-match-sonames libosmocodec0 libosmocore4 libosmogb2 libosmogsm4 libosmovty0
diff --git a/debian/compat b/debian/compat
index 7f8f011..45a4fb7 100644
--- a/debian/compat
+++ b/debian/compat
@@ -1 +1 @@
-7
+8