Fix compilation on Musl
Without this fix, compilation failed on Musl due to ssize_t being an unknown type
In file included from ../src/xrt/state_trackers/prober/p_prober.c:15:
../src/xrt/state_trackers/prober/p_prober.h:67:2: error: unknown type name 'ssize_t'
67 | ssize_t interface;
| ^~~~~~~
Edited by Bart Ribbers