[glibmm] Glib::ustring: Remove extraneous GLIBMM_API

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 7bfe3e56f60d78177252febba5fe481a4421e3d0
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon May 31 15:41:11 2021 +0200

    Glib::ustring: Remove extraneous GLIBMM_API
    
    Deleted methods shall not be decorated with GLIBMM_API.
    Fixes #92

 glib/glibmm/ustring.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/glib/glibmm/ustring.h b/glib/glibmm/ustring.h
index c295fd0b..41165f58 100644
--- a/glib/glibmm/ustring.h
+++ b/glib/glibmm/ustring.h
@@ -981,8 +981,8 @@ class ustring::FormatStream
 {
 public:
   // noncopyable
-  GLIBMM_API FormatStream(const ustring::FormatStream&) = delete;
-  GLIBMM_API FormatStream& operator=(const ustring::FormatStream&) = delete;
+  FormatStream(const ustring::FormatStream&) = delete;
+  FormatStream& operator=(const ustring::FormatStream&) = delete;
 
 private:
 #ifdef GLIBMM_HAVE_WIDE_STREAM