riscv: ptrace: Fix BIT() compilation issues

BIT() is being used in ptrace.h without a definition, resulting in
compilation errors in tools/testing/selftests/riscv/cfi/cfitests.c:

cfitests.c:101:60: error: implicit declaration of function ‘BIT’ [-Wimplicit-function-declaration]
  101 |                      if ((cfi_reg.cfi_status.cfi_state & CFI_ENABLE_MASK) != CFI_ENABLE_MASK)

Include linux/bits.h to resolve this issue.

Fixes: 2af7c9cf021c ("riscv/ptrace: expose riscv CFI status and state via ptrace and in core files")
Signed-off-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Link: https://patch.msgid.link/20260309-fix_selftests-v2-3-9d5a553a531e@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>