MINOR: deviceatlas: define header_evidence_entry in dummy library header

Add the struct header_evidence_entry definition to the dummy dac.h
to accommodate the ongoing deviceatlas module update which now
iterates over atlas header_priorities to precompute maxhdrlen.
The struct was already referenced by struct da_atlas but lacked
a definition in the dummy header.

addons/deviceatlas/dummy/dac.h

@@ -141,6 +141,11 @@ enum {
     DA_INITIAL_MEMORY_ESTIMATE = 1024 * 1024 * 14
 };
 
+struct header_evidence_entry {
+    const char *name;
+    da_evidence_id_t id;
+};
+
 struct da_config {
     unsigned int cache_size;
     unsigned int __reserved[15]; /* enough reserved keywords for future use */