ip address: Fix loop initial declarations are only allowed in C99

On some distros, i.e. rhel 7.6, compilation fails with the following:

ipaddress.c: In function ‘lookup_flag_data_by_name’:
ipaddress.c:1260:2: error: ‘for’ loop initial declarations are only allowed in C99 mode
  for (int i = 0; i < ARRAY_SIZE(ifa_flag_data); ++i) {
  ^
ipaddress.c:1260:2: note: use option -std=c99 or -std=gnu99 to compile your code

This commit fixes the single place needed for compilation to pass.

Fixes: 9d59c86e575b ("iproute2: ip addr: Organize flag properties structurally")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
diff --git a/ip/ipaddress.c b/ip/ipaddress.c
index 3b53933..f97eaff 100644
--- a/ip/ipaddress.c
+++ b/ip/ipaddress.c
@@ -1257,7 +1257,9 @@
 
 /* Returns a pointer to the data structure for a particular interface flag, or null if no flag could be found */
 static const struct ifa_flag_data_t* lookup_flag_data_by_name(const char* flag_name) {
-	for (int i = 0; i < ARRAY_SIZE(ifa_flag_data); ++i) {
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(ifa_flag_data); ++i) {
 		if (strcmp(flag_name, ifa_flag_data[i].name) == 0)
 			return &ifa_flag_data[i];
 	}