tree-wide: add missing header includes

We're missing some header includes leading to missing function
prototypes. While we currently don't warn about these, we should have
their respective headers included in order to detect the case where a
function signature change results in an incompatibility.

Author: pks-t

src/config_snapshot.c

@@ -5,8 +5,9 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include "config.h"
+#include "config_backend.h"
 
+#include "config.h"
 #include "config_entries.h"
 
 typedef struct {

src/reflog.c

@@ -12,7 +12,8 @@
 #include "signature.h"
 #include "refdb.h"
 
-#include <git2/sys/refdb_backend.h>
+#include "git2/sys/refdb_backend.h"
+#include "git2/sys/reflog.h"
 
 void git_reflog_entry__free(git_reflog_entry *entry)
 {

src/streams/registry.c

@@ -5,9 +5,10 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include "git2/errors.h"
-
 #include "common.h"
+
+#include "streams/registry.h"
+
 #include "global.h"
 #include "streams/tls.h"
 #include "streams/mbedtls.h"