rt-tests: fix build of NPTL dependent tests

Commit 143fe5b4 (rt-tests: allow building subset of tests with non-NPTL
toolchains) disabled build of NPTL dependent tests unconditionally. Fix this.

[Thomas:
 - fix typo in commit title.
 - reverse condition, to use positive logic.]

Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Reported-by: "Williams Jr., Ernest L." <ernesto@slac.stanford.edu>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Alexey Brodkin <abrodkin@synpsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>