Make type conversion explicit

This fixes MSVC building warnings.

Change-Id: I71e824027f632858e6f9cf8daf444bb0de649e86
diff --git a/tools/obu_parser.cc b/tools/obu_parser.cc
index 8a608c8..9345b23 100644
--- a/tools/obu_parser.cc
+++ b/tools/obu_parser.cc
@@ -177,7 +177,7 @@
     const size_t length_field_size = kObuLengthFieldSizeBytes;
 #endif  // CONFIG_OBU_SIZING
 
-    obu_overhead += length_field_size;
+    obu_overhead += (int)length_field_size;
 
     if (current_obu_length > remaining) {
       fprintf(stderr,
@@ -186,7 +186,7 @@
               consumed, current_obu_length, remaining);
       return false;
     }
-    consumed += length_field_size;
+    consumed += (int)length_field_size;
 
     obu_header = kEmptyObu;
     const uint8_t obu_header_byte = *(data + consumed);