selftests: breakpoints: fix compile error on breakpoint_test_arm64

The current mainline breakpoints test for arm64 fails to compile with

breakpoint_test_arm64.c: In function ‘set_watchpoint’:
breakpoint_test_arm64.c:97:28: error: storage size of ‘dreg_state’ isn’t known
  struct user_hwdebug_state dreg_state;

Adding a direct include for asm/ptrace.h helps it to build, and passes
the test on mainline on hikey.

Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>