mpv: needs atomics or 8-byte sync

mpv: needs atomics or 8-byte sync

The wscript checks the following.
        'func': check_libs(['atomic'],
            check_statement('stdint.h',
                'int64_t test = 0;'
                'test = __atomic_add_fetch(&test, 1, __ATOMIC_SEQ_CST)')),
and
        'func': check_statement('stdint.h',
                    'int64_t test = 0;'
                    '__typeof__(test) x = ({int a = 1; a;});'
                    'test = __sync_add_and_fetch(&test, 1)'),
It fails if neither of these are available.

Fixes
http://autobuild.buildroot.net/results/8c20f719f784af1312294600e39004c1d382368a

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>