alpine: Fixed runtime depends on libgd.

gd package only holds the binaries linked to libgd.

alpine/Makefile

diff --git a/alpine/Makefile b/alpine/Makefile
index 26083d0..ed44157 100644
--- a/alpine/Makefile
+++ b/alpine/Makefile
@@ -76,7 +76,7 @@
 
 # image-filter
 
-MODULE_DEPENDS_image_filter=gd
+MODULE_DEPENDS_image_filter=libgd
 MODULE_BUILD_DEPENDS_image_filter=gd-dev
 
 define MODULE_ADD_CONTROL_TAGS_image_filter