shout: fix minimum requirement to libshout >= 2.4.3
commit e64c6f0b added usage of the SHOUT_USAGE_UNKNOWN symbol, but this became available in the Icecast-libshout 2.4.3 release, not 2.4.2
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commit e64c6f0b added usage of the SHOUT_USAGE_UNKNOWN symbol, but this became available in the Icecast-libshout 2.4.3 release, not 2.4.2