avfilter/vf_fftfilt: Add missing cpu.h inclusion

Error was: implicit declaration of function 'av_cpu_max_align'
is invalid in C99
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_fftfilt.c

@@ -26,6 +26,7 @@
 
 #include "libavfilter/internal.h"
 #include "libavutil/common.h"
+#include "libavutil/cpu.h"
 #include "libavutil/imgutils.h"
 #include "libavutil/opt.h"
 #include "libavutil/pixdesc.h"