Missing 'const' from reiserfs MIN_KEY declaration.

In stree.c, MIN_KEY is declared const. The extern declaration in dir.c
doesn't match...

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>