libsubcmd: Move EXTRA_FLAGS to the end to allow overriding existing flags

Move EXTRA_WARNINGS and EXTRA_FLAGS to the end of the compilation line,
otherwise they cannot be used to override the default values.

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: nd <nd@arm.com>
Link: http://lore.kernel.org/lkml/20191028113340.4282-1-james.clark@arm.com


[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>