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.