bitvec: Fix left shifting out of range on signed variable

Fixes following ASan runtime errors spotted by TypeTest osmo-pcu unit test:
libosmocore/src/bitvec.c:275:13: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Change-Id: I70502044d05c0505a4b65c1e12e89ff657afe804
diff --git a/src/bitvec.c b/src/bitvec.c
index d7f32fb..2b4e8c9 100644
--- a/src/bitvec.c
+++ b/src/bitvec.c
@@ -272,7 +272,7 @@
 		if (bit < 0)
 			return bit;
 		if (bit)
-			ui |= (1 << (num_bits - i - 1));
+			ui |= ((unsigned)1 << (num_bits - i - 1));
 		bv->cur_bit++;
 	}