BaseTools/Conf: Fix MAKE_FLAGS typos in tools_def.template

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3313



Change MAKE_FLAG to MAKE_FLAGS to match required name from
EDK II Build Specifications for VS20xx tool chains.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Yuwei Chen <yuwei.chen@intel.com>