arm64: stacktrace: make struct stackframe private to stacktrace.c

Now that arm64 uses arch_stack_walk() consistently, struct stackframe is
only used within stacktrace.c. To make it easier to read and maintain
this code, it would be nicer if the definition were there too.

Move the definition into stacktrace.c.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Reviwed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kalesh Singh <kaleshsingh@google.com> for the series.
Link: https://lore.kernel.org/r/20220413145910.3060139-4-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>