OpenBSD has getentropy, but no sys/random.h

https://man.openbsd.org/getentropy

Try to fix https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20210118T023008Z.fail.html.gz
```
compiling random.c
random.c:53:11: fatal error: 'sys/random.h' file not found
# include <sys/random.h>
          ^~~~~~~~~~~~~~
1 error generated.
```

1 files changed, 3 insertions, 1 deletions

diff --git a/random.c b/random.c
index 7b78efe7ca..c11cd803f2 100644
--- a/random.c
+++ b/random.c
@@ -50,7 +50,9 @@
 #endif
 
 #if defined HAVE_GETRANDOM || defined HAVE_GETENTROPY
-# include <sys/random.h>
+# if defined(HAVE_SYS_RANDOM_H)
+#  include <sys/random.h>
+# endif
 #elif defined __linux__ && defined __NR_getrandom
 # include <linux/random.h>
 #endif