Fix ec_test build on Windows.

No need to include unistd.h. (Though it probably should include string.h for
memcmp and strcmp.)

Change-Id: Ib09d2da4f7079c9d87338df75ec3560f4f203764
Reviewed-on: https://boringssl-review.googlesource.com/2260
Reviewed-by: Adam Langley <agl@google.com>

crypto/ec/ec_test.c

@@ -13,7 +13,7 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
 #include <stdio.h>
-#include <unistd.h>
+#include <string.h>
 
 #include <openssl/bio.h>
 #include <openssl/crypto.h>