avutil: Fix compilation with inline asm disabled on mingw

Because of -Werror=implicit-function-declaration the build will fail.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/x86/timer.h

@@ -36,6 +36,7 @@ static inline uint64_t read_time(void)
 
 #elif HAVE_RDTSC
 
+#include <intrin.h>
 #define AV_READ_TIME __rdtsc
 
 #endif /* HAVE_INLINE_ASM */