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 252dc3105fc). Separate it from
the code preceding it (if any) with a blank line.

Backports commit 5f1450f5444b42cde4b1edc61ea5fdcd57404d3e from qemu

qemu/scripts/qapi/common.py

@@ -2226,6 +2226,7 @@ class QAPIGenC(QAPIGenCCode):
 
     def _bottom(self, fname):
         return mcgen('''
+
 /* Dummy declaration to prevent empty .o file */
 char dummy_%(name)s;
 ''',