Fix compilation warning of pg_basebackup with MinGW

Several buildfarm members have been complaining about that with gcc,
like jacana.  Weirdly enough, Visual Studio's compilers do not find this
issue.

Author: Michael Paquier
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/20190719050830.GK1859@paquier.xyz

Author: michaelpq

src/bin/pg_basebackup/pg_basebackup.c

@@ -1445,7 +1445,9 @@ ReceiveAndUnpackTarFile(PGconn *conn, PGresult *res, int rownum)
 
 		if (file == NULL)
 		{
+#ifndef WIN32
 			int			filemode;
+#endif
 
 			/*
 			 * No current file, so this must be the header for a new file
@@ -1459,8 +1461,10 @@ ReceiveAndUnpackTarFile(PGconn *conn, PGresult *res, int rownum)
 
 			current_len_left = read_tar_number(&copybuf[124], 12);
 
+#ifndef WIN32
 			/* Set permissions on the file */
 			filemode = read_tar_number(&copybuf[100], 8);
+#endif
 
 			/*
 			 * All files are padded up to 512 bytes