arm64: stacktrace: Move export for save_stack_trace_tsk()

Due to refactoring way back in bb53c820

 ("arm64: stacktrace: avoid
listing stacktrace functions in stacktrace") the EXPORT_SYMBOL_GPL() for
save_stack_trace_tsk() is at the end of __save_stack_trace() rather than
the function it exports. Move it to the expected location.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200710182402.50473-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>