specify openmp lib in aix-toolchain-include.cpp test

when unset -fopenmp tries to find the library itself and in some
configurations this test fails

fix for #88545 8f07a67f9731dfcd490f8aaefac34d95f207b39c

GitOrigin-RevId: 8363ff6a2e9d20258fb72e4db64ffb6e4167618f
1 file changed