uapi: Fix undefined __always_inline on non-glibc systems

This macro is defined by glibc itself, which makes the issue go unnoticed on
those systems. On non-glibc systems it causes build failures on several
utilities and libraries, like bpftool and objtool.

Fixes: 1d509f2a6ebc ("x86/insn: Support big endian cross-compiles")
Fixes: 2d7ce0e8a704 ("tools/virtio: more stubs")
Fixes: 3fb321fde22d ("selftests/net: ipv6 flowlabel")
Fixes: 50b3ed57dee9 ("selftests/bpf: test bpf flow dissection")
Fixes: 9cacf81f8161 ("bpf: Remove extra lock_sock for TCP_ZEROCOPY_RECEIVE")
Fixes: a4b2061242ec ("tools include uapi: Grab a copy of linux/in.h")
Fixes: b12d6ec09730 ("bpf: btf: add btf print functionality")
Fixes: c0dd967818a2 ("tools, include: Grab a copy of linux/erspan.h")
Fixes: c4b6014e8bb0 ("tools: Add copy of perf_event.h to tools/include/linux/")

Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211115134647.1921-1-ismael@iodev.co.uk
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>