kselftest/arm64: Include <asm/ptrace.h> for user_gcs definition

kselftest/arm64: Include <asm/ptrace.h> for user_gcs definition

kselftest includes kernel uAPI headers with option:

  -isystem $(top_srcdir)/usr/include

Include <asm/ptrace.h> in libc-gcs.c for the definition of struct
user_gcs from the uAPI headers, and remove the redundant definition in
gcs-util.h. This fixes a compilation error on systems where the
toolchain defines NT_ARM_GCS.

Fixes: a505a52b4e29 ("kselftest/arm64: Add a GCS test program built with the system libc")
Signed-off-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>