configure: fix HAVE_GETRANDOM_SYSCALL check

syscall() function requires #include <unistd.h>.
diff --git a/configure b/configure
index 6f26a36..0eff26d 100755
--- a/configure
+++ b/configure
@@ -16276,6 +16276,7 @@
 /* end confdefs.h.  */
 
 
+    #include <unistd.h>
     #include <sys/syscall.h>
 
     int main() {