Windows build fixes.

Windows doesn't have ssize_t, sadly. There's SSIZE_T, but defining an
OPENSSL_SSIZE_T seems worse than just using an int.

Change-Id: I09bb5aa03f96da78b619e551f92ed52ce24d9f3f
Reviewed-on: https://boringssl-review.googlesource.com/1352
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/dh.h b/include/openssl/dh.h
index 84fb456..9d8bda2 100644
--- a/include/openssl/dh.h
+++ b/include/openssl/dh.h
@@ -127,8 +127,8 @@
  * writes it as a big-endian integer into |out|, which must have |DH_size|
  * bytes of space. It returns the number of bytes written, or a negative number
  * on error. */
-OPENSSL_EXPORT ssize_t
-    DH_compute_key(uint8_t *out, const BIGNUM *peers_key, DH *dh);
+OPENSSL_EXPORT int DH_compute_key(uint8_t *out, const BIGNUM *peers_key,
+                                  DH *dh);
 
 
 /* Utility functions. */
@@ -217,8 +217,8 @@
   /* generate_parameters is called by |DH_generate_key|. */
   int (*generate_key)(DH *dh);
 
-  /* generate_parameters is called by |DH_compute_key|. */
-  ssize_t (*compute_key)(DH *dh, uint8_t *out, const BIGNUM *pub_key);
+  /* compute_key is called by |DH_compute_key|. */
+  int (*compute_key)(DH *dh, uint8_t *out, const BIGNUM *pub_key);
 };
 
 struct dh_st {