misc/iio-oscilloscope: fix build on GCC architectures

Build fails with base GCC:
cc1: warnings being treated as errors
/wrkdirs/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.10-master-28-g9b9a441/osc.c: In function 'plugin_data_capture_of_plot':
/wrkdirs/usr/ports/misc/iio-oscilloscope/work/iio-oscilloscope-0.10-master-28-g9b9a441/osc.c:576: warning: 'markers_lock' may be used uninitialized in this function

PR: 241622
Approved by: yuri (maintainer), linimon (mentor)