avcodec/x86/idct_sse2_xvid: fix non C99 inline function

Found-by: Matt Oliver <protogonoi@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/idct_sse2_xvid.c

@@ -343,7 +343,7 @@ DECLARE_ASM_CONST(16, int32_t, walkenIdctRounders)[] = {
     "movdqa   %%xmm6, 4*16("dct")     \n\t" \
     "movdqa   "SREG2", 7*16("dct")    \n\t"
 
-inline void ff_idct_xvid_sse2(short *block)
+av_extern_inline void ff_idct_xvid_sse2(short *block)
 {
     __asm__ volatile(
     "movq     "MANGLE(m127)", %%mm0                              \n\t"