linux-user: Do not define struct sched_attr if libc headers do

linux-user: Do not define struct sched_attr if libc headers do

glibc 2.41+ has added [1] definitions for sched_setattr and
sched_getattr functions and struct sched_attr.  Therefore, it needs
to be checked for here as well before defining sched_attr, to avoid
a compilation failure.

Define sched_attr conditionally only when SCHED_ATTR_SIZE_VER0 is
not defined.

[1] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=21571ca0d70302909cf72707b2a7736cf12190a0;hp=298bc488fdc047da37482f4003023cb9adef78f8

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2799
Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>