Specify commit for realsense-sdk submodule

[ttchalakov]

Need to specify the commit that the realsense-sdk submodule should be on. This is necessary because although the branch is specified correctly, the commit is not, so anytime the submodule is not on the latest commit in the specified branch, git will try stage a change in the local repo to the latest commit. However this is not ideal because a new commit could break functionality in the repo so we do not want to arbitrarily update to the latest repo.

Resources:

• https://git-scm.com/book/en/v2/Git-Tools-Submodules
• https://www.atlassian.com/git/tutorials/git-submodule