knot-resolver: disable jemalloc support

knot-resolver: disable jemalloc support

knot-resolver will attempt to use jemalloc if it's available.
Jemalloc was added to openwrt packages in PR: Add jemalloc and integrate into bind #26721 [1].
At this time, we would rather continue linking with the libc malloc,
so this PR disables jemalloc support explicitly in the knot-resolver build configuration.

[1] https://github.com/openwrt/packages/pull/26721#issuecomment-2973865778

Signed-off-by: Noah Meyerhans <[email protected]>