Fix target remote pipe command for MinGW

Fix target remote pipe command for MinGW

The cced7cacecad104fff0 ("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.