dnsapi: Add a stub for DnsFlushResolverCache.

dlls/dnsapi/dnsapi.spec

diff --git a/dlls/dnsapi/dnsapi.spec b/dlls/dnsapi/dnsapi.spec
index bdc340a..bb14f4b 100644
--- a/dlls/dnsapi/dnsapi.spec
+++ b/dlls/dnsapi/dnsapi.spec
@@ -31,7 +31,7 @@
 @ stdcall DnsExtractRecordsFromMessage_UTF8(ptr long ptr)
 @ stdcall DnsExtractRecordsFromMessage_W(ptr long ptr)
 @ stub DnsFindAuthoritativeZone
-@ stub DnsFlushResolverCache
+@ stdcall DnsFlushResolverCache()
 @ stub DnsFlushResolverCacheEntry_A
 @ stub DnsFlushResolverCacheEntry_UTF8
 @ stub DnsFlushResolverCacheEntry_W