fix: add 'ex' dir required for latex build to work

Author: mnordahl

lectures/latex/ex/.gitignore

@@ -0,0 +1,6 @@
+# Ignore all files, except this .gitignore file.
+# The directory ex must exist for the build to work, but git doesn't
+# track empty directories. Now, it is not empty. (\(^_^)/)
+
+*.*
+!.gitignore