Add missing sundown dependency to rustdoc tests.

Author: metajack

mk/tests.mk

@@ -394,7 +394,8 @@ $(3)/stage$(1)/test/rustdoctest-$(2)$$(X_$(2)):					\
 		$$(RUSTDOC_LIB) $$(RUSTDOC_INPUTS)		\
 		$$(SREQ$(1)_T_$(2)_H_$(3)) \
 		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_LIBSYNTAX_$(2)) \
-		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_LIBRUSTC_$(2))
+		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_LIBRUSTC_$(2)) \
+		$$(SUNDOWN_LIB_$(2))
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(1)_T_$(2)_H_$(3)) -o $$@ $$< --test \
 		-L $$(SUNDOWN_DIR_$(2))