XArray: Include bitmap.h from xarray.h

xas_find_chunk() calls find_next_bit(), which is defined in find.h,
included from bitmap.h.  Inside the kernel, this isn't a problem because
bitmap.h is included from cpumask.h which is dragged in (eventually)
by gfp.h.  When building the test-suite, that doesn't happen, so we need
to include bitmap.h explicitly.

Fixes: 4ade0818 ("tools: sync tools/bitmap with mother linux")
Reported-by: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>