perf probe: Fix 'function unused' warning

perf probe: Fix 'function unused' warning

By 'make build-test' a warning is found in probe-event.c that, after
commit 49694aade2 (perf probe: Show the error reason comes from
invalid DSO) the only user of kernel_get_module_dso() is
open_debuginfo(). Which is not compiled if HAVE_DWARF_SUPPORT not set.

'make build-test' found this problem when make_minimal.

This patch moves kernel_get_module_dso() to HAVE_DWARF_SUPPORT ifdef
section.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1432779905-206143-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>