Realtime tests: Fix bad priority inheritance conditionals

testcases/realtime/lib/librttest.c and
testcases/realtime/stress/pi-tests/testpi-3.c
both referenced a non-existent autoconf configuration setting variable.
Replaced the invalid variable name with the variable actually created
by autoconf.

Signed-off-by: Gary S. Robertson <gary.robertson@linaro.org>
Acked-by: Jan Stancek <jstancek@redhat.com>