devel/cargo-c: unbreak build after 2c68245ea1a7

[libgit2-sys 0.15.2+1.6.4]
The following warnings were emitted during compilation:

warning: libgit2/src/libgit2/transports/ssh.c:11:10: fatal error: 'libssh2.h' file not found
warning: #include <libssh2.h>
warning: ^~~~~~~~~~~
warning: 1 error generated.