meson: Fix exported functions in plugins

This adds DEF file names in meson. Without the DEF files, every
functions are exproted from plugins.

plugins/exec/meson.build

@@ -1,5 +1,6 @@
 shared_module('exec', 'exec.c',
   dependencies: hexchat_plugin_dep,
   install: true,
-  install_dir: plugindir
+  install_dir: plugindir,
+  vs_module_defs: 'exec.def',
 )