Extraneous NULL in linux_target_ops when HAVE_LINUX_BTRACE not defined

This fixes the followin error when HAVE_LINUX_BTRACE is not defined:

    linux-low.c:5943: error: excess elements in struct initializer
    linux-low.c:5943: error: (near initialization for 'linux_target_ops')

gdb/gdbserver/ChangeLog:

        * linux-low.c (linux_target_ops) [!HAVE_LINUX_BTRACE]:
        Remove extraneous NULL element.
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