Fix compatibility with newer Boost.

Extracted from this upstream commit:
https://cgit.freedesktop.org/libreoffice/core/commit/?id=23a8d5ffbbe58761b89f590f0735abccd69a3681

diff --git a/sfx2/source/appl/shutdownicon.cxx b/sfx2/source/appl/shutdownicon.cxx
--- a/sfx2/source/appl/shutdownicon.cxx
+++ b/sfx2/source/appl/shutdownicon.cxx
@@ -144,7 +144,7 @@ bool LoadModule()
 #endif // ENABLE_QUICKSTART_APPLET
     }
     assert(!boost::logic::indeterminate(loaded));
-    return loaded;
+    return bool(loaded);
 }
 
 }