scripts: gen_handles: silence "should it be static?" warnings

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>