ioq: Avoid _nounroll warning on older GCC versions

Some older GCC versions (11.4, 12.3, 13.2) give a false-positive warning
when a loop pragma is attached to a loop that declares a variable:

    warning: ignoring loop annotation

Work around it by hoisting the induction variable declarations.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114691

src/ioq.c

@@ -280,11 +280,12 @@ static uintptr_t ioq_slot_wait(struct ioqq *ioqq, ioq_slot *slot, uintptr_t valu
 	uintptr_t ret;
 
 	// Try spinning a few times before blocking
+	int i, j;
 	_nounroll
-	for (int i = 0; i < 10; ++i) {
+	for (i = 0; i < 10; ++i) {
 		// Exponential backoff
 		_nounroll
-		for (int j = 0; j < (1 << i); ++j) {
+		for (j = 0; j < (1 << i); ++j) {
 			spin_loop();
 		}