Makefile: add qapi.py dependencies

Makefile: add qapi.py dependencies

Commit 427a1a2cb1d35b83b6302886f46289f6d617134d ("qapi: avoid reserved
keywords") modifies qapi.py, which is used by qapi-types.py and other
Python scripts.  Because Makefile has no dependencies for qapi.py the
qapi code generator will not be rerun and the following build error is
produced:

  net/slirp.c: In function ‘net_init_slirp’:
  net/slirp.c:721:50: error: ‘NetdevUserOptions’ has no member named ‘q_restrict’

Fix this issue by adding the missing qapi.py dependencies.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>