gsm: Use correct include for getrandom

"man getrandom" states sys/random.h is required.

Fixes warning below:
 warning: implicit declaration of function ‘getrandom’; did you mean ‘srandom’? [-Wimplicit-function-declaration]
  rc = getrandom(out, len, GRND_NONBLOCK);
       ^~~~~~~~~

Change-Id: I2e73fd018e887893dc5527d6d73644d627eb963a
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 8464090..5d8c834 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -100,7 +100,7 @@
 
 /* FIXME: this can be removed once we bump glibc requirements to 2.25: */
 #if defined(__GLIBC__) && (__GLIBC__ >= 2) && (__GLIBC_MINOR__ >= 25)
-#include <linux/random.h>
+#include <sys/random.h>
 #elif HAVE_DECL_SYS_GETRANDOM
 #include <sys/syscall.h>
 #ifndef GRND_NONBLOCK