libxslt: depend on pkg-config for build on Linux (#78062)
libxslt: depend on pkg-config for build on Linux (#78062)

Fixes:
2021-05-25T19:32:46.5378757Z checking for xml2-config... /home/linuxbrew/.linuxbrew/opt/libxml2/bin/xml2-config
2021-05-25T19:32:46.5380027Z ./configure: line 15402: syntax error near unexpected token `LIBXML,'
2021-05-25T19:32:46.5381168Z ./configure: line 15402: `        PKG_CHECK_MODULES(LIBXML, libxml-2.0 >= $LIBXML_REQUIRED_VERSION,'
mfikes/homebrew-core,zyedidia/homebrew-core,stgraber/homebrew-core,Linuxbrew/homebrew-core,nbari/homebrew-core,Linuxbrew/homebrew-core,stgraber/homebrew-core,chrisfinazzo/homebrew-core,Homebrew/homebrew-core,lemzwerg/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,nandub/homebrew-core,chrisfinazzo/homebrew-core,Moisan/homebrew-core,lemzwerg/homebrew-core,cblecker/homebrew-core,filcab/homebrew-core,nbari/homebrew-core,makigumo/homebrew-core,Moisan/homebrew-core,adam-moss/homebrew-core,mfikes/homebrew-core,sjackman/homebrew-core,uyjulian/homebrew-core,nandub/homebrew-core,sjackman/homebrew-core,JCount/homebrew-core,cblecker/homebrew-core,mbcoguno/homebrew-core,adam-moss/homebrew-core,Homebrew/homebrew-core,makigumo/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,JCount/homebrew-core,filcab/homebrew-core