samples/hw_breakpoint: mark sample_hbp as static

There is a sparse warning shown as below:

samples/hw_breakpoint/data_breakpoint.c:24:19: warning:
symbol 'sample_hbp' was not declared. Should it be static?

Since 'sample_hbp' is only called within data_breakpoint.c,
mark it as static to fix the warning.

Fixes: 44ee6358

 ("percpu: Add __percpu sparse annotations to hw_breakpoint")
Signed-off-by: Chen Jiahao <chenjiahao16@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>