drm/amdkfd: Fix prototype warning for get_process_num_bos

Fix the warning: no previous prototype for 'get_process_num_bos'
[-Wmissing-prototypes]

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>