lib/Kconfig: Mark OF_LIBFDT_OVERLAY as depending on OF_LIBFDT

The overlay code is only useful when OF_LIBFDT is set, so mark it as
depending on that first.

Signed-off-by: Tom Rini <trini@konsulko.com>

lib/Kconfig

@@ -286,6 +286,7 @@ config OF_LIBFDT
 
 config OF_LIBFDT_OVERLAY
 	bool "Enable the FDT library overlay support"
+	depends on OF_LIBFDT
 	default y if ARCH_OMAP2PLUS || ARCH_KEYSTONE
 	help
 	  This enables the FDT library (libfdt) overlay support.