Signature size now uses size_t

Signature size shouldn't have negative values. Make its usage more
consistent.

Test: mma
Change-Id: Ie6cbf49694fcbe0348d6338373423d77d3edfdd4
diff --git a/payload_generator/payload_signer.cc b/payload_generator/payload_signer.cc
index 92313dc..9739052 100644
--- a/payload_generator/payload_signer.cc
+++ b/payload_generator/payload_signer.cc
@@ -352,7 +352,7 @@
 }
 
 bool PayloadSigner::HashPayloadForSigning(const string& payload_path,
-                                          const vector<int>& signature_sizes,
+                                          const vector<size_t>& signature_sizes,
                                           brillo::Blob* out_payload_hash_data,
                                           brillo::Blob* out_metadata_hash) {
   // Create a signature blob with signatures filled with 0.