mstcpip.h is not missing on MinGW.

Remove a small difference between MinGW and MSVC builds which isn't
needed for modern MinGW, noticed in passing.

Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

Author: macdice

src/backend/libpq/pqcomm.c

@@ -69,7 +69,7 @@
 #include <netinet/in.h>
 #include <netinet/tcp.h>
 #include <utime.h>
-#ifdef _MSC_VER					/* mstcpip.h is missing on mingw */
+#ifdef WIN32
 #include <mstcpip.h>
 #endif
 

src/interfaces/libpq/fe-connect.c

@@ -44,9 +44,7 @@
 #endif
 #define near
 #include <shlobj.h>
-#ifdef _MSC_VER					/* mstcpip.h is missing on mingw */
 #include <mstcpip.h>
-#endif
 #else
 #include <sys/socket.h>
 #include <netdb.h>