selftests/binderfs: use the Makefile's rules, not Make's implicit rules

First of all, in order to build with clang at all, one must first apply
Valentin Obst's build fix for LLVM [1]. Once that is done, then when
building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...the following error occurs:

   clang: error: cannot specify -o when generating multiple output files

This is because clang, unlike gcc, won't accept invocations of this
form:

    clang file1.c header2.h

While trying to fix this, I noticed that:

a) selftests/lib.mk already avoids the problem, and

b) The binderfs Makefile indavertently bypasses the selftests/lib.mk
build system, and quitely uses Make's implicit build rules for .c files
instead.

The Makefile attempts to set up both a dependency and a source file,
neither of which was needed, because lib.mk is able to automatically
handle both. This line:

    binderfs_test: binderfs_test.c

...causes Make's implicit rules to run, which builds binderfs_test
without ever looking at lib.mk.

Fix this by simply deleting the "binderfs_test:" Makefile target and
letting lib.mk handle it instead.

[1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/

Fixes: 6e29225a ("binderfs: port tests to test harness infrastructure")
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/filesystems/binderfs/Makefile

@@ -3,6 +3,4 @@
 CFLAGS += $(KHDR_INCLUDES) -pthread
 TEST_GEN_PROGS := binderfs_test
 
-binderfs_test: binderfs_test.c ../../kselftest.h ../../kselftest_harness.h
-
 include ../../lib.mk