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>