fuzzers: clean up header includes

There's multiple headers included in our fuzzers that aren't required at
all. Furthermore, some of them are not available on Win32, causing
builds to fail. Remove them to fix this.

fuzzers/config_file_fuzzer.c

@@ -7,15 +7,9 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include <git2.h>
+#include "git2.h"
 #include "config_backend.h"
 
-#include <stdlib.h>
-#include <stdio.h>
-#include <unistd.h>
-#include <limits.h>
-#include <errno.h>
-
 #define UNUSED(x) (void)(x)
 
 int foreach_cb(const git_config_entry *entry, void *payload)

fuzzers/download_refs_fuzzer.c

@@ -7,11 +7,9 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include <string.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <stdio.h>
-#include <unistd.h>
 
 #include "git2.h"
 #include "git2/sys/transport.h"

fuzzers/packfile_fuzzer.c

@@ -7,11 +7,7 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include <stdbool.h>
-#include <stdint.h>
 #include <stdio.h>
-#include <limits.h>
-#include <unistd.h>
 
 #include "git2.h"
 #include "git2/sys/mempack.h"

fuzzers/standalone_driver.c

@@ -5,11 +5,7 @@
  * a Linking Exception. For full terms see the included COPYING file.
  */
 
-#include <assert.h>
-#include <dirent.h>
 #include <stdio.h>
-#include <stdlib.h>
-#include <sys/types.h>
 
 #include "git2.h"
 #include "fileops.h"