Fix default windows FD_SETSIZE, closes #506

https://support.microsoft.com/en-us/kb/111855 describes
that the user is expected to set the FD_SETSIZE macro
before including winsock2.h

mongoose.c

@@ -95,6 +95,9 @@
 #include <BaseTsd.h>
 typedef SSIZE_T ssize_t;
 #endif
+#ifndef FD_SETSIZE
+#define FD_SETSIZE 1024
+#endif
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <windows.h>