postgresql: move explicit libxml2 include behind version flag

This was introduced in #44083 to fix cross building, where xml2-config
wouldn't run on the host platform. This was fixed upstream two years
later [1], so that from v13 on pkg-config is used before xml2-config is.

Once v12 is EOL, we can remove this entirely.

[1]: 0bc8cebdb8

pkgs/servers/sql/postgresql/generic.nix

@@ -88,7 +88,9 @@ let
 
     buildFlags = [ "world" ];
 
-    env.NIX_CFLAGS_COMPILE = "-I${libxml2.dev}/include/libxml2";
+    # Makes cross-compiling work when xml2-config can't be executed on the host.
+    # Fixed upstream in https://github.com/postgres/postgres/commit/0bc8cebdb889368abdf224aeac8bc197fe4c9ae6
+    env.NIX_CFLAGS_COMPILE = lib.optionalString (olderThan "13") "-I${libxml2.dev}/include/libxml2";
 
     configureFlags = [
       "--with-openssl"