Fix up fontconfig .gitignore

[jdm]

After building it in-tree, I now see this:

# Not currently on any branch.
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   .gitignore
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   m4/libtool.m4
#   m4/ltoptions.m4
#   m4/ltsugar.m4
#   m4/ltversion.m4
#   m4/lt~obsolete.m4

Let's correct that.

[khodzha]

@jdm what is building in-tree? I had that issue too, but after checkout make from %servo_root%/build doesn't produce these files anymore. edit: actually they are gitignored and since .gitignore isn't changed, they are ignored properly

[jdm]

We introduced the fontconfig submodule recently, so we build it as part of the build from our source tree now. That's what I mean by "building in-tree".

[khodzha]

@jdm I guess this issue is somehow fixed since now building doesn't change fontconfig's .gitignore -> no untracked content in fontconfig dir

[SimonSapin]

@jdm, do you still see this? #2040 made git status clean for me, but what I had before was different to it could be a separate issue.