libselinux/utils: fix all the noreturn errors

When building with clang, multiple noreturn issues arise,
for instance:

selabel_partial_match.c:11:1: error: function 'usage' could be declared with attribute 'noreturn' [-Werror,-Wmissing-noreturn]

Fix these.

Signed-off-by: William Roberts <[email protected]>
diff --git a/libselinux/utils/selabel_digest.c b/libselinux/utils/selabel_digest.c
index e4d84a5..49408a0 100644
--- a/libselinux/utils/selabel_digest.c
+++ b/libselinux/utils/selabel_digest.c
@@ -8,7 +8,7 @@
 
 static size_t digest_len;
 
-static void usage(const char *progname)
+static __attribute__ ((__noreturn__)) void usage(const char *progname)
 {
 	fprintf(stderr,
 		"usage: %s -b backend [-d] [-v] [-B] [-i] [-f file]\n\n"