libweston: fix bitshift in weston_idalloc_get_id()

Fixes the following UBSan error:

../../git/weston/libweston/id-number-allocator.c:140:16: runtime error:
left shift of 1 by 31 places cannot be represented in type 'int'

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>