FreeBSD: Static libxml2 build

FreeBSD does not maintain a stable libxml2 version within a given OS
release. The library is not ABI stable so when users update their
packages, they are susceptible to seeing the compilers fail to launch.
By bundling it, we are able to mitigate the issue.

Author: etcwilde

utils/build-presets.ini

@@ -1357,6 +1357,7 @@ llvm-targets-to-build=X86;ARM;AArch64;WebAssembly
 #===------------------------------------------------------------------------===#
 [preset: mixin_freebsd_package_products]
 
+static-libxml2
 foundation
 indexstore-db
 libdispatch