znc: add boost dependency

[p-linnane]

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Fixing broken linkage after #153108:

╰─ brew linkage znc
System libraries:
  /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation
  /usr/lib/libSystem.B.dylib
  /usr/lib/libc++.1.dylib
  /usr/lib/libsasl2.2.dylib
  /usr/lib/libz.1.dylib
Homebrew libraries:
  /opt/homebrew/opt/icu4c/lib/libicudata.74.dylib (icu4c)
  /opt/homebrew/opt/icu4c/lib/libicuuc.74.dylib (icu4c)
  /opt/homebrew/opt/openssl@3/lib/libcrypto.3.dylib (openssl@3)
  /opt/homebrew/opt/openssl@3/lib/libssl.3.dylib (openssl@3)
  /opt/homebrew/opt/python@3.12/Frameworks/Python.framework/Versions/3.12/Python (python@3.12)
Broken dependencies:
  /opt/homebrew/opt/boost/lib/libboost_chrono-mt.dylib (boost)
  /opt/homebrew/opt/boost/lib/libboost_locale-mt.dylib (boost)
  /opt/homebrew/opt/boost/lib/libboost_thread-mt.dylib (boost)

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