selftests/bpf: Fix build error for LoongArch

selftests/bpf: Fix build error for LoongArch

There exists build error when make -C tools/testing/selftests/bpf/
on LoongArch:

  BINARY   test_verifier
In file included from test_verifier.c:27:
tools/include/uapi/linux/bpf_perf_event.h:14:28: error: field 'regs' has incomplete type
   14 |         bpf_user_pt_regs_t regs;
      |                            ^~~~
make: *** [Makefile:577: tools/testing/selftests/bpf/test_verifier] Error 1
make: Leaving directory 'tools/testing/selftests/bpf'

Add missing uapi header for LoongArch to use the following definition:
typedef struct user_pt_regs bpf_user_pt_regs_t;

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1676458867-22052-1-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>