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: 1d509f2a ("x86/insn: Support big endian cross-compiles")
Fixes: 2d7ce0e8 ("tools/virtio: more stubs")
Fixes: 3fb321fd ("selftests/net: ipv6 flowlabel")
Fixes: 50b3ed57 ("selftests/bpf: test bpf flow dissection")
Fixes: 9cacf81f ("bpf: Remove extra lock_sock for TCP_ZEROCOPY_RECEIVE")
Fixes: a4b20612 ("tools include uapi: Grab a copy of linux/in.h")
Fixes: b12d6ec0 ("bpf: btf: add btf print functionality")
Fixes: c0dd9678 ("tools, include: Grab a copy of linux/erspan.h")
Fixes: c4b6014e ("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>