qemu: fix build issue on new hosts with glibc 2.30
This fixes the following error:
TOPDIR/tmp/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/linux-user/syscall.c:254:16: error: static declaration of ‘gettid’ follows non-static declaration
254 | _syscall0(int, gettid)
| ^~~~~~
TOPDIR/tmp/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/linux-user/syscall.c:185:13: note: in definition of macro ‘_syscall0’
185 | static type name (void) \
| ^~~~
In file included from /usr/include/unistd.h:1170,
from TOPDIR/tmp/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/include/qemu/osdep.h:90,
from TOPDIR/tmp/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/linux-user/syscall.c:20:
/usr/include/bits/unistd_ext.h:34:16: note: previous declaration of ‘gettid’ was here
34 | extern __pid_t gettid (void) __THROW;
| ^~~~~~
