Change all uses of restrict to use av_restrict instead.

Defining restrict results - for some compilers - in changing other
uses of the restrict keyword also, e.g. __declspec(restrict) gets
changed to __declspec(__restrict) on MSVC. This causes compilation
failures. Therefore, using a private namespace macro instead is
more reliable and robust.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>