hal: Fix compilation error when flag is not defined

- HAL compilation fails if the flag HFP_ENABLED is not defined.
- Fix the issue by changing the function prototype if the flag
  is undefined.

CRs-Fixed: 678534
Change-Id: I7043082fd337604899666abc1f5a6c331a4421bc
diff --git a/hal/audio_extn/audio_extn.c b/hal/audio_extn/audio_extn.c
index b641bdb..06bcac8 100644
--- a/hal/audio_extn/audio_extn.c
+++ b/hal/audio_extn/audio_extn.c
@@ -56,7 +56,7 @@
                                    struct str_parms *parms);
 #endif
 #ifndef HFP_ENABLED
-void audio_extn_hfp_set_parameters(adev, parms) (0)
+#define audio_extn_hfp_set_parameters(adev, parms) (0)
 #else
 void audio_extn_hfp_set_parameters(struct audio_device *adev,
                                            struct str_parms *parms);