filesystems/mergerfs: Fix build with newer Clang/libc++

The vendored fmt header uses malloc/free without including the
appropriate declarations, causing builds to fail on recent
FreeBSD -CURRENT toolchains.

Add the missing <cstdlib> include to libfuse/include/fmt/format.h.

Notified by: pkg-fallout

(cherry picked from commit e60f6672636217373c1f86a19ccd168ac18c721a)