Fix Iphlpapi.h -> iphlpapi.h

If you cross compile for Windows you will get an error otherwise that the header cannot be found.
See here for code example from microsoft: https://docs.microsoft.com/en-us/windows/desktop/iphlp/creating-a-basic-ip-helper-application

lib/core/libwebsockets.c

@@ -28,7 +28,7 @@
 #ifdef LWS_WITH_IPV6
 #if defined(WIN32) || defined(_WIN32)
 #include <wincrypt.h>
-#include <Iphlpapi.h>
+#include <iphlpapi.h>
 #else
 #include <net/if.h>
 #endif