prc: fix a compilation warning in encryptStream.cxx

dtool/src/prc/encryptStream.cxx

@@ -27,7 +27,7 @@ read_magic(const char *magic, size_t size) {
   char *this_magic = (char *)alloca(size);
   read(this_magic, size);
 
-  if (!fail() && gcount() == size && memcmp(this_magic, magic, size) == 0) {
+  if (!fail() && (size_t)gcount() == size && memcmp(this_magic, magic, size) == 0) {
     _buf.set_magic_length(size);
     return true;
   } else {