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>