Commit 1056d3d2 ("selftests: enforce local header dependency in
lib.mk") added header dependency to the rule, but as the rule uses $^,
the headers are added to the compiler command line.

This can cause unexpected precompiled header files being generated when
compilation fails:

  $ echo { >> openat2_test.c

  $ make
  gcc -Wall -O2 -g -fsanitize=address -fsanitize=undefined  openat2_test.c
    tools/testing/selftests/kselftest_harness.h tools/testing/selftests/kselftest.h helpers.c
    -o tools/testing/selftests/openat2/openat2_test
  openat2_test.c:313:1: error: expected identifier or ‘(’ before ‘{’ token
    313 | {
        | ^
  make: *** [../lib.mk:140: tools/testing/selftests/openat2/openat2_test] Error 1

  $ file openat2_test*
  openat2_test:   GCC precompiled header (version 014) for C
  openat2_test.c: C source, ASCII text

Fix it by filtering out the headers, so that we'll only pass the actual
*.c files in the compiler command line.

Fixes: 1056d3d2 ("selftests: enforce local header dependency in lib.mk")
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>