backports: lib-arc4: Do not build when activated in kernel

Upstream commit d611ecd9295eafa268b5a1390baabefcd5c980cd

Only build the own lib-arc4 version when it is not build as a module or
into the kernel. Without this change it was also build when
CRYPTO_LIB_ARC4 was build as a module in the kernel.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>