Fix narrowing warning (if enabled).

Summary: They're (signed) long long.

Test Plan: built, folly/test

Reviewed By: andrewjcg@fb.com

FB internal diff: D493270

folly/build/generate_varint_tables.py

@@ -73,7 +73,7 @@ def generate(f):
             # 0xff: set corresponding byte in result to 0
             for k in range(d, 4):
                 vals[j] |= 0xff << (8 * k)
-        f.write("  {{0x{1:08x}{0:08x}U, 0x{3:08x}{2:08x}U}},\n".format(*vals))
+        f.write("  {{0x{1:08x}{0:08x}LL, 0x{3:08x}{2:08x}LL}},\n".format(*vals))
 
     f.write("};\n"
             "\n"