unrandom: #define _GNU_SOURCE, for syscall().

This is needed when unrandom.c is compiled on its own.

Change-Id: Ia46e06d267c097e5fa0296092a7270a4cd0b2044
Reviewed-on: https://boringssl-review.googlesource.com/16085
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/fipsmodule/rand/urandom.c

diff --git a/crypto/fipsmodule/rand/urandom.c b/crypto/fipsmodule/rand/urandom.c
index 202f2a4..9a821f7 100644
--- a/crypto/fipsmodule/rand/urandom.c
+++ b/crypto/fipsmodule/rand/urandom.c
@@ -12,6 +12,10 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#if !defined(_GNU_SOURCE)
+#define _GNU_SOURCE  /* needed for syscall() on Linux. */
+#endif
+
 #include <openssl/rand.h>
 
 #if !defined(OPENSSL_WINDOWS) && !defined(OPENSSL_FUCHSIA) && \