multimedia/ffmpeg4: unbreak GLSLANG=on build after d35ea7b74f4a

libavfilter/glslang.cpp:164:5: error: use of undeclared identifier 'assert'

assert(glslang_refcount);
^

PR: 276469
Tested by: vvd
Regressed by: https://github.com/KhronosGroup/glslang/commit/6f9ab3c2deb6
See also: https://github.com/ffmpeg/ffmpeg/commit/246f841b53e1

(cherry picked from commit d5183fa79df24e0716ab928298d161804e8530de)