Add generated timing files to Coq.gitignore (#3367)

These generated files were added in https://github.com/coq/coq/pull/745

Coq.gitignore

@@ -31,3 +31,13 @@ lia.cache
 nia.cache
 nlia.cache
 nra.cache
+
+# generated timing files
+*.timing.diff
+*.v.after-timing
+*.v.before-timing
+*.v.timing
+time-of-build-after.log
+time-of-build-before.log
+time-of-build-both.log
+time-of-build-pretty.log