EASY: silence narrowing warning for signed char

Summary:
`char` is unsigned in some configurations, so an explicit cast is
needed.

Reviewed By: neildhar

Differential Revision: D55997849

fbshipit-source-id: 6fdebbf6e3393c8e22b548da015700be1e41bf77

unittests/VMRuntime/Base64UtilTest.cpp

@@ -148,7 +148,7 @@ TEST_F(Base64UtilTest, DecodeInvalid) {
   CallResult<StringBuilder> builder =
       StringBuilder::createStringBuilder(runtime, outputLength);
 
-  std::array<char, 5> hasNegative = {'A', 'b', 'c', -15, '\0'};
+  std::array<char, 5> hasNegative = {'A', 'b', 'c', (char)-15, '\0'};
   EXPECT_FALSE(base64Decode(createASCIIRef(hasNegative.data()), *builder));
 
   EXPECT_FALSE(base64Decode(createASCIIRef("==="), *builder));