bitint: Fix build [PR121413]

bitint: Fix build [PR121413]

Sorry, my bootstrap failed last night because of this, I've fixed it
up and it bootstrapped/regtested fine overnight, but in the morning
forgot to adjust the patch before committing.

Without this there is
.../gimple-lower-bitint.cc:7678:36: error: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Werror=sign-compare]
 7678 |                       if (min_prec > limb_prec && abi_limb_prec > limb_prec)
      |                           ~~~~~~~~~^~~~~~~~~~~

2025-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/121413
	* gimple-lower-bitint.cc (gimple_lower_bitint): Fix up last
	commit, cast limb_prec to unsigned before comparison.