util: fix "make rpm" when viratomic.h is used

Although src/util/viratomic.h has been added to the repo, up until now
it hasn't been used. Stefan Berger is using it in his proposed dhcp
snooping patches, and an rpm build with those patches failed due to
viratomic.h not being packed up with the rest of the sources.

src/Makefile.am

@@ -79,6 +79,7 @@ UTIL_SOURCES =							\
 		util/threadpool.c util/threadpool.h		\
 		util/uuid.c util/uuid.h				\
 		util/util.c util/util.h				\
+		util/viratomic.h				\
 		util/viraudit.c util/viraudit.h			\
 		util/virauth.c util/virauth.h			\
 		util/virauthconfig.c util/virauthconfig.h	\