perf bpf: Compile dwarf-regs.c if CONFIG_BPF_PROLOGUE is on

regs_query_register_offset() in dwarf-regs.c is required by BPF
prologue.  This patch compiles it if CONFIG_BPF_PROLOGUE is on to avoid
build failure when CONFIG_BPF_PROLOGUE is on but CONFIG_DWARF is not
set.

Signed-off-by: He Kuang <hekuang@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: He Kuang <hekuang@huawei.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1447675815-166222-10-git-send-email-wangnan0@huawei.com


Signed-off-by: Wang Nan <wangnan0@huawei.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>