arch/riscv/riscv_features.c: fix uclibc build

arch/riscv/riscv_features.c: fix uclibc build

Fix the following uclibc build failure raised since version 2.1.4 and
https://github.com/zlib-ng/zlib-ng/commit/6ff8b52cefe56a824fae1d53fdd687bcde2e53c9:

/home/autobuild/autobuild/instance-2/output-1/build/zlib-ng-2.1.6/arch/riscv/riscv_features.c:4:10: fatal error: sys/auxv.h: No such file or directory
    4 | #include <sys/auxv.h>
      |          ^~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/06a7d8e59ec4de7c711d3f4a4624f67b97d78afe

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>