FindSndFile: Link FLAC in static builds

[fwcd]

This should fix some linker errors in static builds on Linux (before vs. after):

/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux-release/lib/libsndfile.a(flac.c.o): in function `flac_close':
flac.c:(.text+0x958): undefined reference to `FLAC__metadata_object_delete'
/usr/bin/ld: flac.c:(.text+0x9d6): undefined reference to `FLAC__stream_encoder_finish'
/usr/bin/ld: flac.c:(.text+0x9e0): undefined reference to `FLAC__stream_encoder_delete'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux-release/lib/libsndfile.a(flac.c.o): in function `flac_write_header':
flac.c:(.text+0xd37): undefined reference to `FLAC__metadata_object_vorbiscomment_entry_from_name_value_pair'
/usr/bin/ld: flac.c:(.text+0xd4a): undefined reference to `FLAC__metadata_object_vorbiscomment_append_comment'
/usr/bin/ld: flac.c:(.text+0xd65): undefined reference to `FLAC__stream_encoder_set_metadata'
/usr/bin/ld: flac.c:(.text+0xd91): undefined reference to `FLAC__stream_encoder_init_stream'
/usr/bin/ld: flac.c:(.text+0xec7): undefined reference to `FLAC__StreamEncoderInitStatusString'
/usr/bin/ld: flac.c:(.text+0xef2): undefined reference to `FLAC__metadata_object_new'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux-release/lib/libsndfile.a(flac.c.o): in function `sf_flac_meta_callback':
flac.c:(.text+0x10ac): undefined reference to `FLAC__metadata_object_vorbiscomment_find_entry_from'
/usr/bin/ld: /home/runner/work/m1xxx/m1xxx/vcpkg/installed/x64-linux-release/lib/libsndfile.a(flac.c.o): in function `flac_enc_init':
flac.c:(.text+0x1d50): undefined reference to `FLAC__stream_encoder_delete'
/usr/bin/ld: flac.c:(.text+0x1d55): undefined reference to `FLAC__stream_encoder_new'
/usr/bin/ld: flac.c:(.text+0x1d70): undefined reference to `FLAC__stream_encoder_set_channels'
/usr/bin/ld: flac.c:(.text+0x1d83): undefined reference to `FLAC__stream_encoder_set_sample_rate'
/usr/bin/ld: flac.c:(.text+0x1d97): undefined reference to `FLAC__stream_encoder_set_bits_per_sample'
/usr/bin/ld: flac.c:(.text+0x1dae): undefined reference to `FLAC__stream_encoder_set_compression_level'