package/syslog-ng: don't fail if systemd service directory already exists

Add -p argument that ignore that specified directory already exists.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
[yann.morin.1998@free.fr:
  - split to its own patch
  - rewrite commit title
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/syslog-ng/syslog-ng.mk

@@ -113,7 +113,7 @@ endef
 # By default syslog-ng installs a .service that requires a config file at
 # /etc/default, so provide one with the default values.
 define SYSLOG_NG_INSTALL_INIT_SYSTEMD
-	mkdir $(TARGET_DIR)/usr/lib/systemd/system/syslog-ng@.service.d
+	mkdir -p $(TARGET_DIR)/usr/lib/systemd/system/syslog-ng@.service.d
 	printf '[Install]\nDefaultInstance=default\n' \
 		>$(TARGET_DIR)/usr/lib/systemd/system/syslog-ng@.service.d/buildroot-default-instance.conf
 endef