src|tests: Address warning -Wstrict-prototypes

Symptom with Clang 15:
> In file included from userinfo.c:20:
> ./userinfo.h:38:27: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
> void invalidate_user_cache(); /* safe to call from signal handler */
>                           ^
>                            void
> [many more]

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/odirect_read.c b/tests/odirect_read.c
index 8fc746b..42d5b79 100644
--- a/tests/odirect_read.c
+++ b/tests/odirect_read.c
@@ -51,7 +51,7 @@ int main(int argc, char** argv) {
 
 #include <stdio.h>
 
-int main() {
+int main(void) {
     fprintf(stderr, "Not supported on this platform.\n");
     return 1;
 }