tests: meta: fix msvc compiler warnings
tests: meta: fix msvc compiler warnings

gstmeta.c(167): warning C4090: 'function': different 'const' qualifiers
gstmeta.c(172): warning C4090: 'function': different 'const' qualifiers
gstmeta.c(211): warning C4090: 'function': different 'const' qualifiers
gstmeta.c(216): warning C4090: 'function': different 'const' qualifiers

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer