.gitignore: Do not ignore *.out files

The sord library contains some test files named *.out, but they are
ignored by the *.out rule.

a.out is the default file name for executables on Linux if no output
filename is specified. All executables that we build carry a name
without extension. Thus no generated file carries an .out extension.
Therefore just remove the *.out ignore rule.

Note: You can list ignored files by running

    git ls-files -i --exclude-standard

.gitignore

@@ -130,7 +130,6 @@ mac/tests/
 
 # Executables
 *.exe
-*.out
 *.app
 
 # Executable without extension