Changes

src/Makefile.am cosmetic: make it easier to extend

To make it easier to add new source files to libosmonetif_la_SOURCES
list, make two cosmetic changes to it:

* List stream.c before stream_{cli,srv}.c, matching the order
  that is used in other similar instances;
* Use $(NULL) construct so that every source-naming line ends
  with a backslash.

Change-Id: I98cf6d592fb66d5f22d486358ccdab06306ae3b0