bridge: only provide proxy ARP when CONFIG_INET is enabled

When IPV4 support is disabled, we cannot call arp_send from
the bridge code, which would result in a kernel link error:

net/built-in.o: In function `br_handle_frame_finish':
:(.text+0x59914): undefined reference to `arp_send'
:(.text+0x59a50): undefined reference to `arp_tbl'

This makes the newly added proxy ARP support in the bridge
code depend on the CONFIG_INET symbol and lets the compiler
optimize the code out to avoid the link error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 95850116

 ("bridge: Add support for IEEE 802.11 Proxy ARP")
Cc: Kyeyoon Park <kyeyoonp@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>