openvswitch: Change dependency from append to definition

openvswitch: Change dependency from append to definition

ovs_libovsdb_depends and ovs_libofproto_depends append the libatomic
dependency. However in these cases these variables were not previously
defined and thus a reader may search the Makefile for the definition.
Therefore change the operator to explicitly define these dependency
variables, rather than append. In addition add a space after operator to
improve readability and conform to other dependency definitions in the
Makefile.

Signed-off-by: Matthew Hagan <mathagan@fb.com>