perf trace: fix MSG_SPLICE_PAGES build error

perf trace: fix MSG_SPLICE_PAGES build error

Our MPTCP CI and Stephen got this error:

    In file included from builtin-trace.c:907:
    trace/beauty/msg_flags.c: In function 'syscall_arg__scnprintf_msg_flags':
    trace/beauty/msg_flags.c:28:21: error: 'MSG_SPLICE_PAGES' undeclared (first use in this function)
       28 |         if (flags & MSG_##n) {           |                     ^~~~
    trace/beauty/msg_flags.c:50:9: note: in expansion of macro 'P_MSG_FLAG'
       50 |         P_MSG_FLAG(SPLICE_PAGES);
          |         ^~~~~~~~~~
    trace/beauty/msg_flags.c:28:21: note: each undeclared identifier is reported only once for each function it appears in
       28 |         if (flags & MSG_##n) {           |                     ^~~~
    trace/beauty/msg_flags.c:50:9: note: in expansion of macro 'P_MSG_FLAG'
       50 |         P_MSG_FLAG(SPLICE_PAGES);
          |         ^~~~~~~~~~

The fix is similar to what was done with MSG_FASTOPEN: the new macro is
defined if it is not defined in the system headers.

Fixes: b848b26c6672 ("net: Kill MSG_SENDPAGE_NOTLAST")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20230626112847.2ef3d422@canb.auug.org.au/
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230626090239.899672-1-matthieu.baerts@tessares.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>