luci-base: rename .gitignore to .placeholder

Signed-off-by: Jo-Philipp Wich <redacted>

diff --git a/modules/luci-base/root/lib/uci/upload/.gitignore b/modules/luci-base/root/lib/uci/upload/.placeholder
similarity index 100%
rename from modules/luci-base/root/lib/uci/upload/.gitignore
rename to modules/luci-base/root/lib/uci/upload/.placeholder