sylpheed: add dependency on libgtk2

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/sylpheed/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_SYLPHEED
 	bool "sylpheed"
+	select BR2_PACKAGE_LIBGTK2
 	help
 	  lightweight and user-friendly e-mail client.
 

package/sylpheed/sylpheed.mk

@@ -13,7 +13,7 @@ SYLPHEED_INSTALL_TARGET_OPT = DESTDIR=$(TARGET_DIR) install
 
 SYLPHEED_CONF_OPT = --disable-gtkspell --program-prefix=""
 
-SYLPHEED_DEPENDENCIES = host-pkg-config
+SYLPHEED_DEPENDENCIES = host-pkg-config libgtk2
 
 $(eval $(call AUTOTARGETS,package,sylpheed))