qcacmn: Fix build error when Green AP disabled

When the Green AP feature is disabled in the Kbuild the following
build issue is observed:

target_if_green_ap.h:59:10: error: 'struct wlan_green_ap_egap_params'
declared inside parameter list [-Werror]
target_if_green_ap.h:59:10: error: its scope is only this definition
or declaration, which is probably not what you want [-Werror]

Fix this by including a forward reference for the struct.

Change-Id: Ia5d867c0ebde17dba0f20c1e97391197ae9dbb77
CRs-Fixed: 2216249

target_if/green_ap/inc/target_if_green_ap.h

@@ -27,6 +27,8 @@
 #include <qdf_status.h>
 #include <wlan_lmac_if_def.h>
 
+struct wlan_green_ap_egap_params;
+
 /**
  * target_if_register_green_ap_tx_ops() - lmac handler to register
  *   green ap tx_ops callback functions