qapi: Emit a blank line before dummy declaration

We emit a dummy variable in each .c file "to shut up OSX toolchain
warnings about empty .o files" (commit 252dc3105f).  Separate it from
the code preceding it (if any) with a blank line.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180828120736.32323-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

scripts/qapi/common.py

@@ -2218,6 +2218,7 @@ class QAPIGenC(QAPIGenCCode):
 
     def _bottom(self, fname):
         return mcgen('''
+
 /* Dummy declaration to prevent empty .o file */
 char dummy_%(name)s;
 ''',