makefile: Add cc.flags.mk to .gitignore list

If user defines some flags then cc.flags.mk
file is generated. This causes tests to exit with 1.

This change help testing delete snapshot feature
where we need to define flag by command:
  export CFLAGS=-DSPDK_ENABLE_SNAPSHOT_DELETION

Change-Id: I6ddd52d6260393785269e7f34a4b65bf95e5c4e2
Signed-off-by: Pawel Kaminski <pawelx.kaminski@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/454457


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>