Include Winsock2 headers

Contributed by @f4bsch in https://github.com/cesanta/mongoose/pull/626
Submitting here with a minor change: MSDN says that ws2ipdef.h should
not be included directly and users should include ws2tcpip.h

PUBLISHED_FROM=1ebe7eac0cf87b0554432b94049b3d58bf7640f7

mongoose.h

@@ -150,8 +150,12 @@
 #ifdef _MSC_VER
 #pragma comment(lib, "ws2_32.lib") /* Linking with winsock library */
 #endif
+
+#include <winsock2.h>
+#include <ws2tcpip.h>
 #include <windows.h>
 #include <process.h>
+
 #ifndef EINPROGRESS
 #define EINPROGRESS WSAEINPROGRESS
 #endif