meson: add -fno-signed-zeros to default flags

It is safe to enable. Even FFmpeg enables this by default.
https://github.com/FFmpeg/FFmpeg/blob/0415bb74c81ab0019e48bd2989ddf48d66918e9e/configure#L7521

meson.build

@@ -285,7 +285,8 @@ test_flags = ['-Wdisabled-optimization',
               '-Wno-sign-compare',
               '-Wno-switch',
               '-Wno-unused-parameter',
-              '-fno-math-errno']
+              '-fno-math-errno',
+              '-fno-signed-zeros']
 
 flags += cc.get_supported_arguments(test_flags)