[LyX/master] Fix wrong forward declaration

[Georg Baum]

commit 0c97d6edc009a968277bda26d2352e7a0868ae33
Author: Georg Baum <b...@lyx.org>
Date:   Mon Nov 16 21:51:30 2015 +0100

    Fix wrong forward declaration
    
    This popped up in cmake monolithic build once. It looks like BufferView is
    included indirectly by some of the other headers (otherwise we would have 
seen
    compile errors for other build configurations as well), bu I'll keep the
    forward declaration since we don't want to depend on this indirect header
    inclusion.

diff --git a/src/MetricsInfo.h b/src/MetricsInfo.h
index 7ad6242..195d705 100644
--- a/src/MetricsInfo.h
+++ b/src/MetricsInfo.h
@@ -21,11 +21,11 @@
 
 #include <string>
 
-class BufferView;
 
 namespace lyx {
 
 namespace frontend { class Painter; }
+class BufferView;
 class Inset;
 class MacroContext;