bitvec: Fix build on older gcc

bitvec.c: In function 'bitvec_unhex':
bitvec.c:389: error: 'for' loop initial declarations are only allowed in C99 mode
bitvec.c:389: note: use option -std=c99 or -std=gnu99 to compile your code
diff --git a/src/bitvec.c b/src/bitvec.c
index f9341b7..2db62ef 100644
--- a/src/bitvec.c
+++ b/src/bitvec.c
@@ -383,10 +383,12 @@
 
 int bitvec_unhex(struct bitvec *bv, const char *src)
 {
+	unsigned i;
 	unsigned val;
 	unsigned write_index = 0;
 	unsigned digits = bv->data_len * 2;
-	for (unsigned i = 0; i < digits; i++) {
+
+	for (i = 0; i < digits; i++) {
 		if (sscanf(src + i, "%1x", &val) < 1) {
 			return 1;
 		}