treewide: Fix dev references to libxml2

pkgs/applications/misc/pgadmin/default.nix

···
   '';
 
   configureFlags = [
-    "--with-libxml2=${libxml2}"
+    "--with-libxml2=${libxml2.dev}"
     "--with-libxslt=${libxslt.dev}"
   ];
 

pkgs/applications/version-management/redmine/default.nix

···
     mkdir -p vendor/cache
     ${stdenv.lib.concatStrings (map (gem: "ln -s ${gem} vendor/cache/${gem.name};") gemspec)}
 
-    bundle config build.nokogiri --use-system-libraries --with-iconv-dir="${libiconv}" --with-xslt-dir="${libxslt.dev}" --with-xml2-dir="${libxml2}"
+    bundle config build.nokogiri --use-system-libraries --with-iconv-dir="${libiconv}" --with-xslt-dir="${libxslt.dev}" --with-xml2-dir="${libxml2.dev}"
 
     bundle install --verbose --local --deployment
 

pkgs/servers/dns/bind/default.nix

···
   configureFlags = [
     "--localstatedir=/var"
     "--with-libtool"
-    "--with-libxml2=${libxml2}"
+    "--with-libxml2=${libxml2.dev}"
     "--with-openssl=${openssl.dev}"
     "--without-atf"
     "--without-dlopen"

pkgs/tools/misc/snapper/default.nix

···
   enableParallelBuilding = true;
 
   NIX_CFLAGS_COMPILE = [
-    "-I${libxml2}/include/libxml2"
+    "-I${libxml2.dev}/include/libxml2"
   ];
 
   postInstall = ''

pkgs/tools/security/clamav/default.nix

···
     "--with-zlib=${zlib.dev}"
     "--with-libbz2-prefix=${bzip2.dev}"
     "--with-iconv-dir=${libiconv}"
-    "--with-xml=${libxml2}"
+    "--with-xml=${libxml2.dev}"
     "--with-openssl=${openssl.dev}"
     "--with-libncurses-prefix=${ncurses.dev}"
     "--with-libcurl=${curl.dev}"

pkgs/tools/security/modsecurity/default.nix

···
     "--with-pcre=${pcre.dev}"
     "--with-apr=${apr.dev}"
     "--with-apu=${aprutil.dev}/bin/apu-1-config"
-    "--with-libxml=${libxml2}"
+    "--with-libxml=${libxml2.dev}"
   ];
 
   outputs = ["out" "nginx"];

pkgs/tools/text/xml/xmlstarlet/default.nix

···
   preConfigure =
     ''
       export LIBXSLT_PREFIX=${libxslt.dev}
-      export LIBXML_PREFIX=${libxml2}
+      export LIBXML_PREFIX=${libxml2.dev}
       export LIBXSLT_LIBS=$(pkg-config --libs libxslt libexslt)
       export LIBXML_LIBS=$(pkg-config --libs libxml-2.0)
     '';