Some compilers and static analyzers warn when non-static (globals) are
not declared in some .h file. Tweak the generated code and add an
`extern` declaration before each device handle definition to make it
look it was declared in a (non-existent) .h file.
This makes the code a bit longer but it is already generated and very
repetitive anyway.
In the typical SOF configuration this silences about 150 sparse
warnings; as many as device handles.
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
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