Return a const char *, fix warnings:
libavfilter/avfilter.c: In function ‘default_filter_name’:
libavfilter/avfilter.c:414:5: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
libavfilter/avfilter.c: At top level:
libavfilter/avfilter.c:419:5: warning: initialization from incompatible pointer type [enabled by default]
libavfilter/avfilter.c:419:5: warning: (near initialization for ‘avfilter_class.item_name’) [enabled by default]