icons: mono: add .gitkeep to fix build

Build fails with a fresh sync, as the "res" folder is missing.

Change-Id: Id7495191b3eaa838d4137afbfce5b27060d8a1fc
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
diff --git a/icons/mono/res/.gitkeep b/icons/mono/res/.gitkeep
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/icons/mono/res/.gitkeep