net: stream: fix TCP references when INET is not enabled

Fix build of net/core/stream.o when CONFIG_INET is not enabled.
Fixes these build errors (sample):

ld: net/core/stream.o: in function `sk_stream_write_space':
(.text+0x27e): undefined reference to `tcp_stream_memory_free'
ld: (.text+0x29c): undefined reference to `tcp_stream_memory_free'
ld: (.text+0x2ab): undefined reference to `tcp_stream_memory_free'
ld: net/core/stream.o: in function `sk_stream_wait_memory':
(.text+0x5a1): undefined reference to `tcp_stream_memory_free'
ld: (.text+0x5bf): undefined reference to `tcp_stream_memory_free'

Fixes: 1c5f2ced ("tcp: avoid indirect call to tcp_stream_memory_free()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20201118194438.674-1-rdunlap@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>