.gitignore: ignore *.tmp files.

This fixes the problem where, while running `redo version-info.sh`, the
repo would always show up as dirty, because redo creates a temp file
named *.tmp. This caused the version code to always have a -dirty tag,
but not when you run version.sh by hand.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

.gitignore

@@ -1,5 +1,6 @@
 # Binaries for programs and plugins
 *~
+*.tmp
 *.exe
 *.dll
 *.so