SELinux: security_read_policy should take a size_t not ssize_t

The len should be an size_t but is a ssize_t.  Easy enough fix to silence
build warnings.  We have no need for signed-ness.

Signed-off-by: Eric Paris <eparis@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index 2cf6708..3ba4feb 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -86,7 +86,7 @@
 int security_mls_enabled(void);
 
 int security_load_policy(void *data, size_t len);
-int security_read_policy(void **data, ssize_t *len);
+int security_read_policy(void **data, size_t *len);
 size_t security_policydb_len(void);
 
 int security_policycap_supported(unsigned int req_cap);