networking/interface.c: reduce bss usage

function                                             old     new   delta
.rodata                                           158918  158950     +32
display_interfaces                                   133     153     +20
UNSPEC_print                                          56      68     +12
pr_ether                                              59      65      +6
static.proc_read                                       4       1      -3
interface_opt_a                                        4       1      -3
in_ether                                             139     136      -3
ifconfig_main                                       1296    1293      -3
if_readlist_proc                                     686     680      -6
ife_print                                           1350    1338     -12
do_if_print                                           46       -     -46
static.buff                                          369     264    -105
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 4/7 up/down: 70/-181)          Total: -111 bytes

# size busybox_old busybox_unstripped
   text    data     bss     dec     hex filename
 751073    3080   14800  768953   bbbb9 busybox_old
 751073    3048   14688  768809   bbb29 busybox_unstripped

1 files changed, 1 insertions, 1 deletions

diff --git a/include/libbb.h b/include/libbb.h
index 3c9f9fe81..999caae0c 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -699,7 +699,7 @@ struct hwtype {
 	int (*activate) (int fd);
 	int suppress_null_addr;
 };
-extern int interface_opt_a;
+extern smallint interface_opt_a;
 int display_interfaces(char *ifname);
 const struct aftype *get_aftype(const char *name);
 const struct hwtype *get_hwtype(const char *name);