ifplugd: remove spurious newline escape

Commit 5ad5d559 (ifplugd: install configuration files unconditionally)
removed the conditional configuration file install, but left an redundant
newline escape, causing the following error:

package/ifplugd/ifplugd.mk:23: *** missing 'endef', unterminated 'define'.  Stop.

Fix this.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>