meson: don't export symbols by default

Only plugin entry points should be exported.

Currently plugins might export more symbols with
the meson build, as we don't have the exports
regexp there that we pass to libtool.

meson.build

@@ -42,6 +42,11 @@ else
   noseh_link_args = []
 endif
 
+# Symbol visibility
+if cc.has_argument('-fvisibility=hidden')
+  add_project_arguments('-fvisibility=hidden', language: 'c')
+endif
+
 cdata = configuration_data()
 
 check_headers = [