samples/hw_breakpoint: fix building without module unloading

__symbol_put() is really meant as an internal helper and is not available
when module unloading is disabled, unlike the previously used symbol_put():

samples/hw_breakpoint/data_breakpoint.c: In function 'hw_break_module_exit':
samples/hw_breakpoint/data_breakpoint.c:73:9: error: implicit declaration of function '__symbol_put'; did you mean '__symbol_get'? [-Werror=implicit-function-declaration]

The hw_break_module_exit() function is not actually used when module
unloading is disabled, but it still causes the build failure for an
undefined identifier. Enclose this one call in an appropriate #ifdef to
clarify what the requirement is. Leaving out the entire exit function
would also work but feels less clar in this case.

Fixes: 910e230d ("samples/hw_breakpoint: Fix kernel BUG 'invalid opcode: 0000'")
Fixes: d8a84d33

 ("samples/hw_breakpoint: drop use of kallsyms_lookup_name()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>