The source file contains a special code-path for Clang, however this path fails
to compile:

  ../lib/igt_halffloat.c:227:7: error: conflicting types for 'igt_half_to_float'
  float igt_half_to_float(const uint16_t *h, float *f, unsigned int num)
        ^
  ../lib/igt_halffloat.h:26:6: note: previous declaration is here
  void igt_half_to_float(const uint16_t *h, float *f, unsigned int num);
       ^
  1 error generated.

This commit fixes this mismatch.

Signed-off-by: Simon Ser <simon.ser@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>