c: Avoid ICE with _BitInt(N) : 0 bitfield [PR113740]

finish_struct already made sure not to call build_bitint_type for
signed _BitInt(2) : 1;
or
signed _BitInt(2) : 0;
bitfields (but instead build a zero precision integral type,
we remove it later), this patch makes sure we do it also for
unsigned _BitInt(1) : 0;
because of the build_bitint_type assertion that precision is
>= (unsigned ? 1 : 2).

2024-02-05  Jakub Jelinek  <jakub@redhat.com>

	PR c/113740
	* c-decl.cc (finish_struct): Only use build_bitint_type if
	bit-field has width larger or equal to minimum _BitInt
	precision.

	* gcc.dg/bitint-85.c: New test.