Use package libfontconfig1, instead of libfontconfig

[dhoppe]

What this PR does / why we need it:
Use the correct package name. The package libfontconfig does not exist at Debian and Ubuntu:

vagrant@debian-8:~$ apt search libfontconfig
Sorting... Done
Full Text Search... Done
libfontconfig1/oldstable,oldstable 2.11.0-6.3+deb8u1 amd64
  generic font configuration library - runtime

libfontconfig1-dbg/oldstable,oldstable 2.11.0-6.3+deb8u1 amd64
  generic font configuration library - debugging symbols

libfontconfig1-dev/oldstable,oldstable 2.11.0-6.3+deb8u1 amd64
  generic font configuration library - development

The installation only works because apt automatically corrects the package name:

vagrant@debian-8:~$ sudo apt install libfontconfig
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'libfontconfig1' instead of 'libfontconfig'
The following extra packages will be installed:
  fontconfig-config fonts-dejavu-core
The following NEW packages will be installed:
  fontconfig-config fonts-dejavu-core libfontconfig1
0 upgraded, 3 newly installed, 0 to remove and 94 not upgraded.
Need to get 0 B/1,650 kB of archives.
After this operation, 3,879 kB of additional disk space will be used.
Do you want to continue? [Y/n]

Special notes for your reviewer:
The dependency has already been correctly configured in the following file:

• https://github.com/grafana/grafana/blob/master/scripts/build/ci-build/Dockerfile#L86

[bergquist]

Thank you for contributing to Grafana! :)