stubs: Optimize dependencies for gdbstub.c

It does not need qemu-common.h. Including exec/gdbstub.h fixes a warning
from static code analyzers and avoids mismatching declarations for
xml_builtin.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

stubs/gdbstub.c

-#include "qemu-common.h"
+#include "stdbool.h"            /* bool (in exec/gdbstub.h) */
+#include "stddef.h"             /* NULL */
+#include "exec/gdbstub.h"       /* xml_builtin */
 
 const char *const xml_builtin[][2] = {
   { NULL, NULL }