C source and header files are not getting downloaded by go get

[sithembiso]

This was causing the following error (see go-git/go-git#624 for more details) when building a project that depends on this library:

# github.com/pjbgf/sha1cd/cgo
vendor/github.com/pjbgf/sha1cd/cgo/sha1.go:3:11: fatal error: 'lib/sha1.h' file not found
 #include <lib/sha1.h>
          ^~~~~~~~~~~~
1 error generated.

This was as a result of missing .c and .h files that go get omits. I have used the embed directive to include these files during the download.

[pjbgf]

Thank you @sithembiso for your PR. I ended up merging #7 instead as that was submitted first and fixed the issue.

[sithembiso]

@pjbgf No problem. I was going to withdraw this one anyway because it introduces a feature that is not available in older versions on Go.