bunzip2: fix code bloat caused by zcat's seamless magic

This example single-applet configuration would trigger the bloat:

    CONFIG_FEATURE_SEAMLESS_XZ=y
    CONFIG_FEATURE_SEAMLESS_LZMA=y
    CONFIG_FEATURE_SEAMLESS_BZ2=y
    CONFIG_FEATURE_SEAMLESS_GZ=y
    CONFIG_BUNZIP2=y
    # CONFIG_ZCAT is not set
    # All other applets disabled

Here, the resulting "busybox-bunzip2" binary would contain
unpack_gz_stream, unpack_lzma_stream and unpack_xz_stream functions
code. In other words, the gzip, lzma and xz decompressors' code are
linked into the binary unnecessarily.

This happens because SEAMLESS_MAGIC != 0 and compiler is unable
to figure out that SEAMLESS_MAGIC bit is never set.

Fix this by disabling SEAMLESS_MAGIC option flag (setting its value
to 0) when zcat is disabled. This will help the compiler optimize out
bbunpack() and no longer generate open_zipped() function call.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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