treewide: Fix dev references to libxml2

pkgs/tools/misc/snapper/default.nix

@@ -42,7 +42,7 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   NIX_CFLAGS_COMPILE = [
-    "-I${libxml2}/include/libxml2"
+    "-I${libxml2.dev}/include/libxml2"
   ];
 
   postInstall = ''

pkgs/tools/security/clamav/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
     "--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

@@ -22,7 +22,7 @@ stdenv.mkDerivation rec {
     "--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

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
   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)
     '';