src: use include/osmocom/abis instead of include/osmocom/gsm/abis

Harald prefers short paths.

This patch also remove commented includes in several files.
diff --git a/include/osmocom/Makefile.am b/include/osmocom/Makefile.am
index 999a3ee..f9d5e7c 100644
--- a/include/osmocom/Makefile.am
+++ b/include/osmocom/Makefile.am
@@ -1 +1 @@
-SUBDIRS=gsm
+SUBDIRS=abis