selftests/bpf: Fix compilation failure for prog vrf_socket_lookup

When building the latest kernel/selftest with clang17 compiler:

make LLVM=1 -j <== for kernel
make -C tools/testing/selftests/bpf LLVM=1 -j <== for selftest

I hit the following compilation error:

[...]
In file included from progs/vrf_socket_lookup.c:3:
In file included from /usr/include/linux/ip.h:21:
In file included from /usr/include/asm/byteorder.h:5:
In file included from /usr/include/linux/byteorder/little_endian.h:13:
/usr/include/linux/swab.h:136:8: error: unknown type name '__always_inline'
136 | static __always_inline unsigned long __swab(const unsigned long y)
| ^
/usr/include/linux/swab.h:171:8: error: unknown type name '__always_inline'
171 | static __always_inline __u16 __swab16p(const __u16 *p)
| ^
/usr/include/linux/swab.h:171:29: error: expected ';' after top level declarator
171 | static __always_inline __u16 __swab16p(const __u16 *p)
| ^
[...]

Basically, with header files in my local host which is based on 5.12 kernel,
__always_inline is not defined and this caused compilation failure.

Since __always_inline is defined in bpf_helpers.h, let us move bpf_helpers.h
to an early position which fixed the problem.

Signed-off-by: Yonghong Song <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/vrf_socket_lookup.c

···
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
 #include <linux/ip.h>
 #include <linux/in.h>
 #include <linux/if_ether.h>
 #include <linux/pkt_cls.h>
-#include <bpf/bpf_helpers.h>
-#include <bpf/bpf_endian.h>
 #include <stdbool.h>
 
 int lookup_status;