random-util: use ssize_t for getrandom return value

This matches the prototype provided by glibc.

src/basic/missing_syscall.h

@@ -78,7 +78,8 @@ static inline int missing_memfd_create(const char *name, unsigned int flags) {
 /* ======================================================================= */
 
 #if !HAVE_GETRANDOM
-static inline int missing_getrandom(void *buffer, size_t count, unsigned flags) {
+/* glibc says getrandom() returns ssize_t */
+static inline ssize_t missing_getrandom(void *buffer, size_t count, unsigned flags) {
 #  ifdef __NR_getrandom
         return syscall(__NR_getrandom, buffer, count, flags);
 #  else