lib: fix sparse shadowed variable warning

pos is always set before being used, no need to declare a
second one inside the if() block.

lib/prio_heap.c:34:7: warning: symbol 'pos' shadows an earlier one
lib/prio_heap.c:30:6: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>