build: quiet generation of include/spdk/config.h

A recent commit accidentally removed the leading $(Q)
when generating include/spdk/config.h.  Add it back to
reduce clutter when building SPDK.

Fixes: b41f404d (make: Fix python command in Makefile)

Signed-off-by: Jim Harris <james.r.harris@intel.com>

Change-Id: I2084d1e0b24a6a3eafc7c6aef201ca3b8ddd181c
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/449


Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Tested-by:  <sys_sgci@intel.com>
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/452


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>