[fix] extern C for ffmpeg includes

[CrystalP]

Description

Changed ffmpeg includes that were not already surrounded with extern "C".

Motivation and context

C functions should be enclosed in extern "C" blocks for correctness, even though linker seems able to work around when missing.

How has this been tested?

Windows 10 x64 and UWP x64. The code linked before and after.
No difference in program behavior before / after light testing

What is the effect on users?

none

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[garbear]

👍

RP changes look good.