build: update .gitignore

Add .\Symbols\* to the list for directories Git ought to ignore.

We'll use the new directory as a place to aggregate symbols prior to publishing them.

.gitignore

@@ -6,6 +6,7 @@
 [Oo]utput/
 [Pp]ackages/
 [Rr]elease/
+[Ss]ymbols/
 [Tt]est[Rr]esult/
 
 *.suo