ignore security alert for libgit2

[syphar]

Fixes #2414 :

libgit2 is used by rustwide:

$ cargo tree -i libgit2-sys                                                   
libgit2-sys v0.15.2+1.6.4
└── git2 v0.17.2
    └── rustwide v0.16.0
        └── docs-rs v0.6.0 (/Users/syphar/src/rust-lang/docs.rs)

and from what I see, the affected methods are not used.