fix: correct regexp.h include path

Changed #include "regexp.h" to #include "smartdns/regexp.h"
to avoid conflict with glibc's deprecated <regexp.h>.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: weiping

src/dns_conf/dns_conf.c

@@ -49,7 +49,7 @@
 #include "proxy_server.h"
 #include "ptr.h"
 #include "qtype_soa.h"
-#include "regexp.h"
+#include "smartdns/regexp.h"
 #include "server.h"
 #include "server_group.h"
 #include "smartdns_domain.h"