qmake/vcxproj: Fix malformed <Message> tags

Extra compilers and the command set to "\n" would result in malformed
<Message> tags in vcxproj files.  Those tags are used to display the
name of the extra compiler when building.  Setting the extra compiler's
command to "\n" is a common trick to force the creation of the rule.

Make sure to trim the command name that is created from the extra
compiler's command to avoid such new-line-only bogus message tags.

Pick-to: 6.2 5.15
Change-Id: I1bae28ed14c438d777f96280c6b2cf5ca315b51c
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

qmake/generators/win32/msvc_objectmodel.cpp

@@ -2437,6 +2437,7 @@ bool VCFilter::addExtraCompiler(const VCFilterFile &info)
                 cmd_name = cmd.left(space);
             else
                 cmd_name = cmd;
+            cmd_name = cmd_name.trimmed();
         }
 
         // Fixify paths