rules.mk: remove "$(STAGING_DIR)/include"

rules.mk: remove "$(STAGING_DIR)/include"

"$(STAGING_DIR)/include" was carried over from buildroot-ng to OpenWrt
in commit 60c1f0f64d23003a19a07d6b9638542130f6641d. buildroot has
dropped this directory a long time ago.

In OpenWrt the directory is still created by the PrepareStaging macro
and is part of the default TARGET_CPPFLAGS. But nothing at all installs
headers into this directory, nor should anything be installed under this
path.

Removing this directory from TARGET_CPPFLAGS will cut down the log noise
a bit. Not only will CPPFLAGS be shorter, there will be less warnings
set off by "-Wmissing-include-dirs" (or even failures when paired with
"-Werror"). After all the directory does not even _exist_ in the SDKs,
which are used on the build bots when building packages (see [1] and
[2]).

make[8]: Entering directory '/builder/shared-workdir/build/sdk/build_dir/target-aarch64_generic_musl/libmbim-1.20.0/src/common'
  CC       libmbim_common_la-mbim-common.lo
cc1: error: /builder/shared-workdir/build/sdk/staging_dir/target-aarch64_generic_musl/include: No such file or directory [-Werror=missing-include-dirs]
cc1: all warnings being treated as errors

[1] https://github.com/openwrt/packages/issues/10377
[2] https://github.com/openwrt/packages/pull/10378

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
Acked-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Rosen Penev <rosenp@gmail.com>