CLEANUP: Add haproxy/xxhash.h to avoid modifying import/xxhash.h

This solves setting XXH_INLINE_ALL in a cleaner way, because the imported
header is not modified, easing future updates.

see 6f7cc11e6dd0f01b437fba893da2edd2362660a2
diff --git a/admin/dyncookie/dyncookie.c b/admin/dyncookie/dyncookie.c
index 0c778eb..ddb71a7 100644
--- a/admin/dyncookie/dyncookie.c
+++ b/admin/dyncookie/dyncookie.c
@@ -14,7 +14,8 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <arpa/inet.h>
-#include <import/xxhash.h>
+
+#include <haproxy/xxhash.h>
 
 __attribute__((noreturn)) void die(int code, const char *format, ...)
 {