jack2: fix build with gcc 6

jack2: fix build with gcc 6

With gcc version 6, we get the following error when building jack2:

  ../tests/iodelay.cpp:170:49: error: narrowing conversion of '-1' from 'int' to 'jack_nframes_t {aka unsigned int}' inside { } [-Wnarrowing]
   jack_latency_range_t   capture_latency = {-1, -1};
                                                   ^
  ../tests/iodelay.cpp:170:49: error: narrowing conversion of '-1' from 'int' to 'jack_nframes_t {aka unsigned int}' inside { } [-Wnarrowing]
  ../tests/iodelay.cpp:171:50: error: narrowing conversion of '-1' from 'int' to 'jack_nframes_t {aka unsigned int}' inside { } [-Wnarrowing]
   jack_latency_range_t   playback_latency = {-1, -1};
                                                    ^
  ../tests/iodelay.cpp:171:50: error: narrowing conversion of '-1' from 'int' to 'jack_nframes_t {aka unsigned int}' inside { } [-Wnarrowing]

This is fixed by grabbing an upstream commit,
ff1ed2c4524095055140370c1008a2d9cccc5645

Fixes:

  http://autobuild.buildroot.net/results/8a8/8a8d533a0f785591fee10f1c09c9294f892ef7f7/

Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>