linux-user/syscall.c: fix build without RLIMIT_RTTIME

linux-user/syscall.c: fix build without RLIMIT_RTTIME

RLIMIT_RTTIME is not provided by uclibc-ng or by musl prior to version
1.2.0 and
https://github.com/bminor/musl/commit/2507e7f5312e79620f6337935d0a6c9045ccba09
resulting in the following build failure since
https://git.qemu.org/?p=qemu.git;a=commit;h=244fd08323088db73590ff2317dfe86f810b51d7:

../linux-user/syscall.c: In function 'target_to_host_resource':
../linux-user/syscall.c:1057:16: error: 'RLIMIT_RTTIME' undeclared (first use in this function); did you mean 'RLIMIT_NOFILE'?
 1057 |         return RLIMIT_RTTIME;
      |                ^~~~~~~~~~~~~
      |                RLIMIT_NOFILE

Fixes:
 - http://autobuild.buildroot.org/results/22d3b584b704613d030e1ea9e6b709b713e4cc26

Signed-off-by: Fabrice Fontaine <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Message-Id: <20220523105239.1499162[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>