databases/mongodb60: unbreak on 15 + llvm 19.1.2

stick to <= llvm18 on FreeBSD 15

fixes:
[00:05:15] In file included from src/third_party/mozjs/platform/aarch64/freebsd/build/Unified_cpp_js_src13.cpp:2:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/ArrayBufferObject.cpp:7:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/ArrayBufferObject-inl.h:13:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/ArrayBufferObject.h:20:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/Runtime.h:55:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/Caches.h:22:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/JSScript.h:46:
[00:05:15] In file included from src/third_party/mozjs/extract/js/src/vm/SharedImmutableStringsCache.h:22:
[00:05:15] src/third_party/mozjs/extract/js/src/threading/ExclusiveData.h:113:29: error: reference to non-static member function must be called
[00:05:15] 113 | : lock_(std::move(rhs.lock)), value_(std::move(rhs.value_)) {
[00:05:15] | ~~~~^~~~