Changes

include/osmocom/netif/Makefile.am cosmetic: make it easier to extend

To make it easier to add new headers to osmonetif_HEADERS list,
make two cosmetic changes to it:

* List all currently exported headers in alphabetical order;
* Use $(NULL) construct so that every header-naming line ends
  with a backslash.

Change-Id: I05ecbdb44286b842e158278fb07479846cd66806