lib: Makefile: build fdtdec_common.c when OF_LIBFDT selected

When build SPL_OF_PLATDATA on i.MX6, meet issue the fdtdec_get_int
not defined, however fdtdec.c will use fdtdec_get_int, so let's
compile fdtdec_common.c when OF_LIBFDT selected.

Since there is also SPL_OF_LIBFDT, so need to use
CONFIG_$(SPL_TPL_)OF_LIBFDT.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/lib/Makefile b/lib/Makefile
index 09c45b8..2fffd68 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -88,7 +88,7 @@
 obj-$(CONFIG_CRC32C) += crc32c.o
 obj-y += ctype.o
 obj-y += div64.o
-obj-$(CONFIG_OF_LIBFDT) += fdtdec.o
+obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += fdtdec.o fdtdec_common.o
 obj-y += hang.o
 obj-y += linux_compat.o
 obj-y += linux_string.o