qcacmn: Fix spectral scan compilation issue

Forward declare 'struct wmi_spectral_cmd_ops' in
file wlan_spectral_utils_api.h to avoid
compilation issue when spectral feature flag
is disabled.

Change-Id: Iba8dfbb2007f20d409fbb2d67ee29f702264e50d
CRs-Fixed: 2269005

spectral/dispatcher/inc/wlan_spectral_utils_api.h

@@ -25,6 +25,7 @@
 
 /* Forward declaration */
 struct direct_buf_rx_data;
+struct wmi_spectral_cmd_ops;
 
 /**
  * wlan_spectral_init() - API to init spectral component