nptl: don't include tcb-offset in lowlevellock.c

The generated tcb-offset.h header file is inteded for assembly files
that cannot resolve the sizeof() operator, and (should) not be needed
in a C file.

The line was added to lowlevellock.c with f14c5848, but doesn't use any
of the definitions of the tcb-offset.sym.

Signed-off-by: Chris Zankel <chris@zankel.net>