lavfi/showspectrum: fix outpicref initialization.

In 81f2549, output pixel format was changed from rgb24 to planar yuv,
but the initialization was left unchanged. As a result, the chroma
planes were left uninitalized. This was not noticed since the current
ff_get_video_buffer() seems to be initializing the buffer. This won't be
the case anymore after the Evil Plan.
Revert "build: disable iconv by default."

This reverts commit 9ad3cd5b5f848eb3cd033be515d34296f099148b.

The misdetection on OS X should be fixed, we will only get
problem reports if auto-detection is enabled.