.gitignore: Ignore LSP generated .cache

Language servers like clangd generate a .cache directory to store
compilation cache. Tell Git to ignore this directory when staging files.

Reviewed by:	markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D38937

Author: jakesfreeland

.gitignore

@@ -25,5 +25,6 @@ cscope.po.out
 compile_commands.json
 compile_commands.events.json
 tags
+.cache
 .clangd
 .ccls-cache