stress: honor the BR2_PREFER_STATIC_LIB preference

Stress is linked statically if the --enable-static is specified.
However, this option is always specified in the global
SHARED_STATIC_LIBS_OPTS to tell packages to build static libraries,
if supported.

If the BR2_PREFER_STATIC_LIB is not defined, we have to specify
--disable-static explicitly to get stress linked dynamically.

Signed-off-by: Tzu-Jung Lee <tjlee@ambarella.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>