package/i2pd: really install configuration files

While applying [1] the commit "fffbd2c9f7 i2pd: new package"
unintentionally removed completely the post-install hooks, so
I2PD_INSTALL_CONFIGURATION_FILES is declared but never called.

Fix the install of configuration files by adding the defined symbol to
the post-install hooks.

[1] http://patchwork.ozlabs.org/project/buildroot/patch/20180327212401.5173-1-fontaine.fabrice@gmail.com/

Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/i2pd/i2pd.mk

@@ -44,6 +44,7 @@ define I2PD_INSTALL_CONFIGURATION_FILES
 	mkdir -p $(TARGET_DIR)/var/lib/i2pd
 	cp -a $(@D)/contrib/certificates $(TARGET_DIR)/var/lib/i2pd
 endef
+I2PD_POST_INSTALL_TARGET_HOOKS += I2PD_INSTALL_CONFIGURATION_FILES
 
 define I2PD_USERS
 	i2pd -1 i2pd -1 * /var/lib/i2pd - - I2P Daemon