media: venus: place extern venus_fw_debug on a header file

Sparse warns about this symbol:

	drivers/media/platform/qcom/venus/hfi_venus.c:133:5:  warning: symbol 'venus_fw_debug' was not declared. Should it be static?

Because hfi_venus.c doesn't include a header file with the
extern. So, move it to core.h, with is included by both
hfi_venus.c and dbgfs.c.

This way, if something changes with it, warnings or errors
will be produced.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>