system.h: fix MinGW build

CURLSYS_PULL_WS2TCPIP_H got renamed to CURL_PULL_WS2TCPIP_H in commit
73a2fcea0b.

include/curl/system.h

@@ -438,9 +438,9 @@
 # define CURL_TYPEOF_CURL_SOCKLEN_T int
 #endif
 
-/* CURLSYS_PULL_WS2TCPIP_H is defined above when inclusion of header file  */
+/* CURL_PULL_WS2TCPIP_H is defined above when inclusion of header file  */
 /* ws2tcpip.h is required here to properly make type definitions below. */
-#ifdef CURLSYS_PULL_WS2TCPIP_H
+#ifdef CURL_PULL_WS2TCPIP_H
 #  ifndef WIN32_LEAN_AND_MEAN
 #    define WIN32_LEAN_AND_MEAN
 #  endif