bitmap: fix problems in previous commit

bitmap: fix problems in previous commit

Commit ee3d3893 missed the fact that (unsigned char)<<(int)
is truncated to int, and therefore failed for any bitmap data
longer than four bytes.

Also, I failed to run 'make syntax-check' on my commit 4bba6579;
for whatever odd reason, ffs lives in a different header than ffsl.

* src/util/bitmap.c (virBitmapNewData): Use correct shift type.
(includes): Glibc (and therefore gnulib) decided ffs is in
<strings.h>, but ffsl is in <string.h>.
* tests/virbitmaptest.c (test5): Test it.