bpf, libbpf: Avoid unused function warning on bpf_tail_call_static

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: Ian Rogers <irogers@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113223609.3358812-1-irogers@google.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))