devel/cargo-c: unbreak build with libgit2 1.8.1

warning: libgit2-sys@0.16.2+1.7.2: failed to probe system libgit2:
[...]
warning: libgit2-sys@0.16.2+1.7.2: libgit2/src/libgit2/transports/ssh.c:11:10: fatal error: 'libssh2.h' file not found
warning: libgit2-sys@0.16.2+1.7.2: 11 | #include <libssh2.h>
warning: libgit2-sys@0.16.2+1.7.2: | ^~~~~~~~~~~
warning: libgit2-sys@0.16.2+1.7.2: 1 error generated.

PR: 280706
Reported by: eduardo