Makefile: use override directive on engine CFLAGS

Makefile: use override directive on engine CFLAGS

The GNU Make documentation says the following about the override
keyword:

  Subsequent assignments or appends to this variable which are not
  marked override will be ignored.

  https://www.gnu.org/software/make/manual/html_node/Override-Directive.html

When the override CFLAGS line was moved in commit
f4bd2c3d80bc35f76892205a7e50426711e3def3 it broke the engine template,
which does not use the override directive. Since the engine template is
now evaluated afterwards, the engine CFLAGS are ignored.

Add the override keyword to the engine template so that engine CFLAGS
are honored again.

Note that the CONFIG_DYNAMIC_ENGINES case doesn't need the override
directive. It seems that GNU Make assigns CFLAGS even without the
override directive in this case:

  $$($(1)_OBJS): CFLAGS := -fPIC $$($(1)_CFLAGS) $(CFLAGS)

Fixes: f4bd2c3d80bc35f76892205a7e50426711e3def3 ("fix dynamic engine build")
Cc: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>