build fix for recent c++ compilers.

thanks, compiler writers, for helping us avoid this heinous sin:

src/vcdecoder_test.cc:33:3: error: narrowing conversion of '195' from 'int' to 'char' inside { } [-Wnarrowing]

and the diff used to avoid this:

const char VCDiffDecoderTest::kStandardFileHeader[] = {
-    0xD6,  // 'V' | 0x80
-    0xC3,  // 'C' | 0x80
-    0xC4,  // 'D' | 0x80
-    0x00,  // Draft standard version number
+    (char)0xD6,  // 'V' | 0x80
+    (char)0xC3,  // 'C' | 0x80
+    (char)0xC4,  // 'D' | 0x80
+    (char)0x00,  // Draft standard version number
    0x00  // Hdr_Indicator: no custom code table, no compression
  };

(agc)