avfilter/vf_convolution: Fix build failures

avfilter/vf_convolution: Fix build failures

98e419cb added SIMD for the convolution filter for x64 systems. As
usual, it used a check of the form
if (ARCH_X86_64)
    ff_convolution_init_x86(s);
and thereby relied on the compiler eliminating this pseudo-runtime check
at compiletime for non x64 systems (for which ff_convolution_init_x86
isn't defined) to compile. But vf_convolution.c contains more than one
filter and if the convolution filter is disabled, but one of the other
filters (prewitt, sobel, roberts) is enabled, the build will fail on x64,
because ff_convolution_init_x86 isn't defined in this case.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>