<sys/bitstring.h>: Cast _BITSTR_BITS to int in a ternary operator.
This fixes a -Wsign-compare error reported by GCC due to the two
results of the ternary operator having differing signedness.
Reviewed by: dougm, rlibby
Differential Revision: https://reviews.freebsd.org/D34122
(cherry picked from commit 8a67a1a9646ef516fe84ecfa7ec3467b15b9e0cc)