multimedia/ffmpeg: unbreak VULKAN=off after 685e477b807c

ERROR: libplacebo >= 4.192.0 not found using pkg-config

/usr/local/include/libplacebo/vulkan.h:21:10: fatal error: 'vulkan/vulkan.h' file not found
#include <vulkan/vulkan.h>

^~~~~~~~~~~~~~~~~

PR: 261302
Reported by: Matthew Wener