[DL Streamer] Add opencv build flag

[mholowni]

Description

OpenCV flag build flag was added for performance improvement.

Any Newly Introduced Dependencies

N/A

How Has This Been Tested?

Performance was tested using dls inside docker container

Checklist:

• I agree to use the APACHE-2.0 license for my code changes.
• I have not introduced any 3rd party components incompatible with APACHE-2.0.
• I have not included any company confidential information, trade secret, password or security token.
• I have performed a self-review of my code.

[brmarkus]

The title (but no detailed description) says "add opencv build flag", but in fact the flag WITH_IPP got added.

Could you add more details to title/description or write something like "add opencv-related build flag WITH_IPP due to/because of...", please?

What about non-Intel environments, would IPP be available on e.g. ARM systems, would the flag work or would it be ignored (as not supported)?