mathops/x86: work around inline asm miscompilation with GCC 4.8.1

The volatile is not required here, and prevents a miscompilation with GCC
4.8.1 when building on x86 with --cpu=i686
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/mathops.h

@@ -74,7 +74,7 @@ static av_always_inline av_const int64_t MUL64(int a, int b)
 static inline av_const int mid_pred(int a, int b, int c)
 {
     int i=b;
-    __asm__ volatile(
+    __asm__ (
         "cmp    %2, %1 \n\t"
         "cmovg  %1, %0 \n\t"
         "cmovg  %2, %1 \n\t"