Add missing headers

[BhavikaSharma]

Two changes:

1. Add #include <cinttypes> to test/cctest/cctest.h. Without it, on Linux systems, the following error occurs:

note: 'PRId64' is defined in header '<cinttypes>'; did you forget to '#include <cinttypes>'?

and similarly for PRIu64 macro.

2. double-conversion/bignum-dtoa.h added as a required header in CMakeLists.txt.

double-conversion/bignum-dtoa.cc is added to the library which references this header.

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[floitsch]

Thanks!
Since the change is small enough, no need to sign the CLA.