build: Don't ignore qapi-visit-core.c

This file is version-controlled, and not generated from a .json file.

Fixes: bf582c34Reported-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190912184607.3507-1-eblake@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

.gitignore

@@ -41,6 +41,7 @@
 /qapi/qapi-types-*.[ch]
 /qapi/qapi-types.[ch]
 /qapi/qapi-visit-*.[ch]
+!/qapi/qapi-visit-core.c
 /qapi/qapi-visit.[ch]
 /qapi/qapi-doc.texi
 /qemu-doc.html