Update libgit2 submodule to include case-sensitive FS fix

[mersinvald]

Hi, currently libgit2 fails to build on case-sensitive file systems (like ext4) when cross-compiling for Windows.

cargo:warning=libgit2/src/path.c:17:10: fatal error: AclAPI.h: No such file or directory
cargo:warning= #include <AclAPI.h>
cargo:warning=          ^~~~~~~~~~
cargo:warning=compilation terminated.

In libgit2 upstream there's a fix for this error already: libgit2/libgit2@44d5e47#diff-6268fdcd3d7ea1e368576fa1736e5b95

[nossralf]

This should be fixed now, I believe, as #503 has been merged.

[alexcrichton]

Yay!