vdpau: use ctz()

(cherry picked from commit 105af824e6e5d298fa797bee2515a4fd121f1b39)
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>

modules/codec/avcodec/vdpau.c

@@ -97,7 +97,7 @@ static int Lock (vlc_va_t *va, AVFrame *ff)
         return VLC_EGENERIC;
     }
 
-    unsigned idx = __builtin_ctz (sys->available);
+    unsigned idx = ctz (sys->available);
     sys->available &= ~(1 << idx);
 
     VdpVideoSurface *surface = sys->surfaces + idx;