.gitignore: Add compilation database file

This file is used by clangd to use language server protocol.
It can be generated at each compile using scripts/gen_compile_commands.py.
Therefore it is different depending on the environment and should be
ignored.

Signed-off-by: Toru Komatsu <k0ma@utam0k.jp>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>