Add indent.c to .gitignore

This is because of the make rule 'indent' which runs 'indent' on the
files, saving to 'indent.c'.

If any entry ever has a file called indent.c something will have to
change obviously but for now this will prevent a cluttered git status if
someone runs make indent (like I did yesterday or the day before).

.gitignore

@@ -38,3 +38,4 @@ mkentry
 .su[a-z]
 .*.sv[a-z]
 .*.sw[a-z]
+indent.c