[PARISC] Fix CONFIG_DEBUG_SPINLOCK

Joel Soete points out that we refer to pa_tlb_lock but only define it if
CONFIG_SMP which breaks a uniprocessor build with CONFIG_DEBUG_SPINLOCK
enabled.  No module refers to pa_tlb_lock, so we can delete the export.

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>