KVM: selftests: elf: Include <endian.h> instead of <bits/endian.h>

<bits/endian.h> is a glibc-internal header that explicitly states it
should never be included directly:

  #error "Never use <bits/endian.h> directly; include <endian.h> instead."

Replace it with the correct public header <endian.h> which works on
all C libraries including musl. Building KVM selftests with musl-gcc
fails with:

  lib/elf.c:10:10: fatal error: bits/endian.h: No such file or directory

Fixes: 6089ae0 ("kvm: selftests: add sync_regs_test")
Signed-off-by: Hisam Mehboob <hisamshar@gmail.com>
Message-ID: <20260409164020.1575176-4-hisamshar@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: hissamshar

tools/testing/selftests/kvm/lib/elf.c

@@ -7,7 +7,7 @@
 
 #include "test_util.h"
 
-#include <bits/endian.h>
+#include <endian.h>
 #include <linux/elf.h>
 
 #include "kvm_util.h"