qca-wifi-host-cmn: Add void keyword to old-style zero prototype functions

Newer clang builds complain about old-style function declarations
not having any prototype assigned. The missing `void` is triggering
the Wstrict-prototype warns. Explicitly add the `void` keyword to
satisfy the compiler.

This fixes the following warnings:

drivers/staging/qca-wifi-host-cmn/hif/src/ce/ce_service_legacy.c:
1288:34: error: a function declaration without a prototype is
deprecated in all versions of C [-Werror,-Wstrict-prototypes]
struct ce_ops *ce_services_legacy()
                                 ^
                                  void
drivers/staging/qca-wifi-host-cmn/hif/src/ce/ce_service_srng.c:1017:32:
error: a function declaration without a prototype is deprecated in all
versions of C [-Werror,-Wstrict-prototypes]
struct ce_ops *ce_services_srng()
                               ^
                                void
drivers/staging/qca-wifi-host-cmn/target_if/core/src/target_if_main.c:
88:40: error: a function declaration without a prototype is
deprecated in all versions of C [-Werror,-Wstrict-prototypes]
struct target_if_ctx *target_if_get_ctx()
                                       ^
                                        void
drivers/staging/qca-wifi-host-cmn/umac/scan/dispatcher/src/wlan_extscan_api.c:
103:27: error: a function declaration without a prototype is
deprecated in all versions of C [-Werror,-Wstrict-prototypes]
wlan_extscan_global_deinit()
                          ^
                           void

Change-Id: Ifd95b57d19a25f0932e5de8ec4de2f2c1e7e1259
Signed-off-by: Tashfin Shakeer Rhythm <tashfinshakeerrhythm@gmail.com>