Fix target remote pipe command for MinGW

The cced7cacec ("gdb: preserve `|` in connection details string")
commit added '|' detection and removal to ser-pipe.c, but missed to add it
to ser-mingw.c.

This results in the error message below for MinGW hosts:
error starting child process '| <executable> <args>': CreateProcess: No such file or directory

This commit add the missing '|' detection and removal to ser-mingw.c.

(cherry picked from commit c43d829bca)

gdb/ser-mingw.c

@@ -867,6 +867,12 @@ pipe_windows_open (struct serial *scb, const char *name)
   if (name == NULL)
     error_no_arg (_("child command"));
 
+  if (*name == '|')
+    {
+      name++;
+      name = skip_spaces (name);
+    }
+
   gdb_argv argv (name);
 
   if (! argv[0] || argv[0][0] == '\0')