(Hopefully) fix a warning on Windows.

MSVC unhelpfuly says: warning C4146: unary minus operator applied to
unsigned type, result still unsigned.

Change-Id: Ia1e6b9fc415908920abb1bcd98fc7f7a5670c2c7

crypto/ec/p256-x86_64.c

diff --git a/crypto/ec/p256-x86_64.c b/crypto/ec/p256-x86_64.c
index a1f91bf..7a56ece 100644
--- a/crypto/ec/p256-x86_64.c
+++ b/crypto/ec/p256-x86_64.c
@@ -138,7 +138,7 @@
 
 static void copy_conditional(BN_ULONG dst[P256_LIMBS],
                              const BN_ULONG src[P256_LIMBS], BN_ULONG move) {
-  BN_ULONG mask1 = -move;
+  BN_ULONG mask1 = ((BN_ULONG)0) - move;
   BN_ULONG mask2 = ~mask1;
 
   dst[0] = (src[0] & mask1) ^ (dst[0] & mask2);