gitignore: the C build folder a self compile leaves behind

A self compile parks its generated C in .<binary>.v3cc.<token> next to the binary and
does not always remove it, so a 73 MB folder shows up as untracked after `./v self`.

Author: HuntedByTheIRS

.gitignore

@@ -174,3 +174,6 @@ issue*
 .copilot/
 .agents/
 .codex/
+
+# the C build folder a self compile leaves behind (driver.v names it .<binary>.v3cc.<token>)
+.*.v3cc.*