mpd: fix mpd.conf installation check

If target /etc/mpd.conf exists the evaluation will be fail thus breaking
the build process. Fix it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/multimedia/mpd/mpd.mk

@@ -96,9 +96,10 @@ MPD_CONF_OPT += --disable-wavpack
 endif
 
 define MPD_INSTALL_EXTRA_FILES
-	[ ! -f $(TARGET_DIR)/etc/mpd.conf ] && \
+	@if [ ! -f $(TARGET_DIR)/etc/mpd.conf ]; then \
 		$(INSTALL) -D package/multimedia/mpd/mpd.conf \
-			$(TARGET_DIR)/etc/mpd.conf
+			$(TARGET_DIR)/etc/mpd.conf; \
+	fi
 	$(INSTALL) -m 0755 -D package/multimedia/mpd/S95mpd \
 		$(TARGET_DIR)/etc/init.d/S95mpd
 endef