arm neon: Add missing mangle to external symbol

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23418 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/arm/synth_filter_neon.S

@@ -36,7 +36,7 @@ function ff_synth_filter_float_neon, export=1
         mov             r9,  r1                 @ synth_buf
 
 VFP     vpush           {d0}
-        bl              ff_imdct_half_neon
+        bl              X(ff_imdct_half_neon)
 VFP     vpop            {d0}
         pop             {r3}