libssh2-sys: use src instead .git as vendored indicator

[catap]

When someone vendored libssh-sys he may exclude .git folder.

When such things happened an attempt to build it may lead to error like: fatal: not a git repository (or any of the parent directories): .git.

[demosdemon]

This will resolve #288 once committed.

[yodaldevoid]

The error may be printed, but it should not stop compilation unless the vendored copy of libssh2 really hasn't been populated.

[kiripon]

Hi, @yodaldevoid .

I would like this PR to be merged too.

I build Rust using a different build system than cargo, which tracks and logs all file accesses made during the build to determine if a build needs to be rerun.
Unless this change is merged, it will be assumed that the build dependency is accessing .git/index, which will affect the build rerun decision.