selftests: vDSO: use KHDR_INCLUDES for UAPI headers for getrandom test

Building test_vdso_getrandom currently leads to following issue:

    In file included from /home/xry111/git-repos/linux/tools/include/linux/compiler_types.h:36,
                     from /home/xry111/git-repos/linux/include/uapi/linux/stddef.h:5,
                     from /home/xry111/git-repos/linux/include/uapi/linux/posix_types.h:5,
                     from /usr/include/asm/sigcontext.h:12,
                     from /usr/include/bits/sigcontext.h:30,
                     from /usr/include/signal.h:301,
                     from vdso_test_getrandom.c:14:
    /home/xry111/git-repos/linux/tools/include/linux/compiler-gcc.h:3:2: error: #error "Please don't include <linux/compiler-gcc.h> directly, include <linux/compiler.h> instead."
        3 | #error "Please don't include <linux/compiler-gcc.h> directly, include <linux/compiler.h> instead."
          |  ^~~~~

It's because the compiler_types.h inclusion in
include/uapi/linux/stddef.h is expected to be removed by the
header_install.sh script, as compiler_types.h shouldn't be used from
user space.

Add KHDR_INCLUDES before the existing include/uapi inclusion so that
usr/include takes precedence if it's populated.
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tools/testing/selftests/vDSO/Makefile

@@ -39,6 +39,7 @@ $(OUTPUT)/vdso_test_correctness: LDFLAGS += -ldl
 
 $(OUTPUT)/vdso_test_getrandom: parse_vdso.c
 $(OUTPUT)/vdso_test_getrandom: CFLAGS += -isystem $(top_srcdir)/tools/include \
+                                         $(KHDR_INCLUDES) \
                                          -isystem $(top_srcdir)/include/uapi
 
 $(OUTPUT)/vdso_test_chacha: $(top_srcdir)/tools/arch/$(ARCH)/vdso/vgetrandom-chacha.S