kbuild: avoid build error when single DTB is turned into composite DTB

As commit afa974b7 ("kbuild: add real-prereqs shorthand for
$(filter-out FORCE,$^)") explained, $(real-prereqs) is not just a list
of objects when linking a multi-object module. If a single-object module
is turned into a multi-object module, $^ (and therefore $(real-prereqs)
as well) contains header files recorded in the *.cmd file. Such headers
must be filtered out.

Now that a DTB can be built either from a single source or multiple
source files, the same issue can occur.

Consider the following scenario:

First, foo.dtb is implemented as a single-blob device tree.

The code looks something like this:

[Sample Code 1]

  Makefile:

      dtb-y += foo.dtb

  foo.dts:

    #include <dt-bindings/gpio/gpio.h>
    /dts-v1/;
    / { };

When it is compiled, .foo.dtb.cmd records that foo.dtb depends on
scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h.

Later, foo.dtb is split into a base and an overlay. The code looks
something like this:

[Sample Code 2]

  Makefile:

      dtb-y += foo.dtb
      foo-dtbs := foo-base.dtb foo-addon.dtbo

  foo-base.dts:

    #include <dt-bindings/gpio/gpio.h>
    /dts-v1/;
    / { };

  foo-addon.dtso:

    /dts-v1/;
    /plugin/;
    / { };

If you rebuild foo.dtb without 'make clean', you will get this error:

    Overlay 'scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h' is incomplete

$(real-prereqs) contains not only foo-base.dtb and foo-addon.dtbo but
also scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h, which is
passed to scripts/dtc/fdtoverlay.

Fixes: 15d16d6d

 ("kbuild: Add generic rule to apply fdtoverlay")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>