bpf, libbpf: Avoid unused function warning on bpf_tail_call_static

mainline inclusion
from mainline-5.12-rc1
commit ce5a518e
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce5a518e9de53446f10d46fac98640f7ac026100

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Add inline to __always_inline making it match the linux/compiler.h.
Adding this avoids an unused function warning on bpf_tail_call_static
when compining with -Wall.
Signed-off-by: NIan Rogers <irogers@google.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113223609.3358812-1-irogers@google.com
(cherry picked from commit ce5a518e)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

tools/lib/bpf/bpf_helpers.h

@@ -30,7 +30,7 @@
 #define SEC(NAME) __attribute__((section(NAME), used))
 
 #ifndef __always_inline
-#define __always_inline __attribute__((always_inline))
+#define __always_inline inline __attribute__((always_inline))
 #endif
 #ifndef __noinline
 #define __noinline __attribute__((noinline))