net: wireless: bcmdhd: ifdef wl_cfg80211_sched_scan_stop declaration

wl_cfg80211_sched_scan_stop() was introduced in
4e98e4ad9f77dffa3e6d977453f92259d0370e3d "net: wireless: bcmdhd: cancel
current PNO if connection will happen" but its declaration was not
ifdef'd like its usage is. Picky compilers are picky.

Change-Id: Ibd97b98c99e2b0bb2bf933b7ba858181f346e1de