Fix: USB/Serial/Parallel Port transport add-ons link error

Hi guy(s?),

Today I discover that 3 of our print transports were wrong. They didn't=20
export init_transport() and exit_transport() !
As we were linking them against the static libprinttransportaddon.a but=20
without refering these symbols from the addon, the linki process didn't=20
include them.

So, I just commit a fix to this issue : we now explicitly link with=20
PrintTransportAddOn.o instead.

I've renamed these transport add-ons main source file from=20
print_transport.cpp to *Transport.cpp to remove a potential jam names=20
conflict.=20
And I think these new names make more sense.

Oh my, how I'm bored these days! ;-)

- Philippe=20

--
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