powerpc/udbg: Remove extern function prototypes

'extern' keyword is pointless and deprecated for function prototypes.

Signed-off-by: Pali Rohár <pali@kernel.org>
Suggested-by: Gabriel Paubert <paubert@iram.es>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220822231751.16973-1-pali@kernel.org