htl: Fix build error in annexc

We were getting

../scripts/evaluate-test.sh posix/annexc $? true false > /usr/src/glibc-upstream/build/posix/annexc.test-result
In file included from ../include/pthread.h:1,
                 from <stdin>:1:
../sysdeps/htl/include/pthread.h:7:62: error: missing binary operator before token "("
    7 | # if defined __USE_EXTERN_INLINES && defined _LIBC && !IS_IN (libsupport)
      |                                                              ^