Mk/Uses/cargo.mk: Fix incorrect linker argument in RUSTFLAGS

According to the manpage of rustc(1) it is: link-args='-flag1 -flag2'
an 's' was missing.

Approved by: jrm (mentor), otis (mentor), mikael (rust maintainer)
MFH: 2025Q3
Differential Revision: https://reviews.freebsd.org/D51803