src: avoid warning about strncpy without trailing NUL

We know that the destination will already have a trailing NUL after the
three characters we're copying. The compiler can't see this though and
warns about lack of NUL termination.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/windows-cmdline-wrapper.c

@@ -62,7 +62,7 @@ int main(int argc, char *argv[])
         // We expect our helper to end with .com
         assert(strncmp(name + len - 3, "com", 4) == 0);
         // replace .com with .exe
-        strncpy(name + len - 3, "exe", 3);
+        memcpy(name + len - 3, "exe", 3);
     }
 
     si.cb = sizeof(si);