databases/mongodb70: unbreak on 15 + llvm 19.1.2

stick to <= llvm18 on FreeBSD 15

fixes:
In file included from src/third_party/mozjs/platform/x86_64/freebsd/build/Unified_cpp_js_src4.cpp:20:
In file included from src/third_party/mozjs/extract/js/src/jsapi.cpp:42:
In file included from src/third_party/mozjs/extract/js/src/gc/PublicIterators.h:17:
In file included from src/third_party/mozjs/extract/js/src/gc/Zone.h:33:
In file included from src/third_party/mozjs/extract/js/src/vm/JSScript.h:41:
In file included from src/third_party/mozjs/extract/js/src/vm/SharedImmutableStringsCache.h:22:
src/third_party/mozjs/extract/js/src/threading/ExclusiveData.h:113:29: error: reference to non-static member function must be called

113 |       : lock_(std::move(rhs.lock)), value_(std::move(rhs.value_)) {
    |                         ~~~~^~~~

1 error generated.