AMDGPU: gfx906: usage of omp_get_device_num() gives warning

[kasaurov]

Compilation of a code with omp_get_device_num() gives warning about implicit declaration of the function, while compiled on gfx906 only (e.g. https://github.com/ROCm-Developer-Tools/aomp/tree/master/test/smoke/devices).
Although this is only a warning, the test can be compiled and executed.

devices.c:19:25: warning: implicit declaration of function 'omp_get_device_num' is invalid in C99 [-Wimplicit-function-declaration]

[ronlieb]

This turns out to be the header file issue. which copy of omp.h being found in include search path affected the warning …