Fix build on older systems

Make sure GRND_NONBLOCK is always defined, even when using syscall
directly.

Change-Id: I1bcac37ee1847596b49122f9307bd2689ba71b1b
Related: OS#1694
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index f572c64..e3f792e 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -101,6 +101,9 @@
 #include <linux/random.h>
 #elif HAVE_DECL_SYS_GETRANDOM
 #include <sys/syscall.h>
+#ifndef GRND_NONBLOCK
+#define GRND_NONBLOCK 0x0001
+#endif
 #endif
 
 /* ETSI GSM 03.38 6.2.1 and 6.2.1.1 default alphabet