lib/interval_tree.c: simplify includes

The file uses nothing from init.h, and also doesn't need the full module.h
machinery; export.h is sufficient.  The latter requires the user to ensure
compiler.h is included, so do that explicitly instead of relying on some
other header pulling it in.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>