.gitignore: simplify due to meson

There is a lot of cruft left in the .gitignore from when this
repository was built using autotools and when it included libpldm,
so simplify it.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ice4f5fea1d0e19bea71643a56df06df547763642