llist: Add a C++ wrapper for linux_list

This commit adds the LListHead class which is a wrapper around the
linuxlist. It adds an additional member to refer to the container,
since the container_of macro doesn't work properly with C++ classes.
All functions and macros from linuxlist.h are support except for the
entry macros (e.g. llist_entry, llist_for_each_entry, ...). To access
the container (entry), an entry() method is provided instead:

  llist_for_each(pos, &elems) {
      pos->entry()->do_something();
  }

Sponsored-by: On-Waves ehf
diff --git a/src/Makefile.am b/src/Makefile.am
index 8a422f6..0398028 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -95,7 +95,8 @@
 	rlc.h \
 	decoding.h \
 	llc.h \
-	pcu_utils.h
+	pcu_utils.h \
+	cxx_linuxlist.h
 
 osmo_pcu_SOURCES = pcu_main.cpp