[DNSAPI][REACTOS] Add a header for undocumented dnsapi functions and use it in dnsapi.dll

sdk/include/reactos/windns_undoc.h

@@ -0,0 +1,30 @@
+#ifndef _WINDNS_UNDOC_H_
+#define _WINDNS_UNDOC_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct _DNS_CACHE_ENTRY
+{
+    struct _DNS_CACHE_ENTRY *pNext; /* Pointer to next entry */
+    PWSTR pszName;                  /* DNS Record Name */
+    unsigned short wType;           /* DNS Record Type */
+    unsigned short wUnknown;        /* Unknown */
+    unsigned short wFlags;          /* DNS Record Flags */
+} DNSCACHEENTRY, *PDNSCACHEENTRY;
+
+BOOL
+WINAPI
+DnsFlushResolverCache(VOID);
+
+BOOL
+WINAPI
+DnsGetCacheDataTable(
+    _Out_ PDNSCACHEENTRY *DnsCache);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _WINDNS_UNDOC_H_ */