kmemleak: Enable the building of the memory leak detector

This patch adds the Kconfig.debug and Makefile entries needed for
building kmemleak into the kernel.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
diff --git a/mm/Makefile b/mm/Makefile
index ec73c68..cb84a02 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -38,3 +38,4 @@
 endif
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
+obj-$(CONFIG_DEBUG_KMEMLEAK) += kmemleak.o