riscv: vdso_cfi: Add .gitignore for build artifacts

riscv: vdso_cfi: Add .gitignore for build artifacts

The vdso_cfi build process copies source files (*.c, *.S) from the main
vdso directory to the build directory. Without a .gitignore file, these
copied files appear as untracked files in git status, cluttering the
working directory.

Add a .gitignore file to exclude:
- Copied source files (*.c, *.S)
- Temporary build files (vdso.lds, *.tmp, vdso-syms.S)
- While preserving vdso-cfi.S which is the original entry point

This follows the same pattern used in the main vdso directory
and keeps the working directory clean.

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Link: https://patch.msgid.link/20260320021850.1877-3-cp0613@linux.alibaba.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>