contrib/plugins/uftrace_symbols.py: ignore zero sized symbols

When using llvm-addr2line in replacement of addr2line, it will output
zero sized symbols, which can shadow other binaries depending on where
their location is (happens with arm-trusted-firmware and its different
binaries). Thus, ignore those symbols.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260306051553.2778652-1-pierrick.bouvier@linaro.org
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>