graphics/mesa-devel: unbreak on -CURRENT after GNU qsort_r

In file included from ../src/util/u_qsort.cpp:24:
../src/util/u_qsort.h:68:4: error: no matching function for call to 'qsort_r'

qsort_r(base, nmemb, size, &data, util_qsort_adapter);
^~~~~~~

/usr/include/stdlib.h:314:7: note: candidate function not viable: no known conversion from 'struct util_qsort_adapter_data *' to 'int (*)(const void *, const void *, void *)' for 4th argument
void qsort_r(void *, size_t, size_t,

^

PR: 266227
Reported by: antoine (via exp-run)
Submitted by: delphij