meson: Always build with hidden symbol visibility

This copylib isn't intended to be part of anyone's ABI. In Autotools,
this is typically implemented by doing some ad-hoc compiler checks and
adding -fvisibility=hidden to the global CFLAGS of any library that
pulls in libglnx, but in Meson it's a build system feature.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -83,6 +83,7 @@ libglnx_sources = [
 libglnx = static_library('glnx',
   libglnx_sources,
   dependencies : libglnx_deps,
+  gnu_symbol_visibility : 'hidden',
   include_directories : libglnx_inc,
   install : false)
 libglnx_dep = declare_dependency(