hostkey.c: Fix 'unsigned int'/'uint32_t' mismatch (#461)

File: hostkey.c

Notes: 
These types are the same size so most compilers are fine with it, but CodeWarrior (on classic MacOS) throws an ‘illegal implicit conversion’ error

Credit: Brendan Shanks

src/hostkey.c

@@ -647,7 +647,7 @@ hostkey_method_ssh_ecdsa_sig_verify(LIBSSH2_SESSION * session,
 {
     unsigned char *r, *s, *name;
     size_t r_len, s_len, name_len;
-    unsigned int len;
+    uint32_t len;
     struct string_buf buf;
     libssh2_ecdsa_ctx *ctx = (libssh2_ecdsa_ctx *) (*abstract);