tools/Makefile: suppress "which swig" error output

The Makefile in tools/ tries to find the "swig" utility by calling "which".
If nothing is found in the path, some versions of which will print an error
message:
$ make clean
which: no swig in (/usr/local/bin:/usr/bin:/bin)

This does not apply to all version of "which", though:
$ echo $0
bash
$ type which
which is aliased to `type -path'
$ which foo				<== this version is OK
$ /usr/bin/which foo			<== this one is chatty
/usr/bin/which: no foo in (/usr/local/bin:/usr/bin:/bin)
$ sh					<== make uses /bin/sh
sh-4.3$ which foo			<== no alias here
which: no foo in (/usr/local/bin:/usr/bin:/bin)

This error message is rather pointless in our case, since we just have
this very check to care for this. So add stderr redirection to suppress
the message.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>