Using deprecated abseil APIs

[GyrosGeier]

When compiling against abseil 20260526:

/build/reproducible-path/prjxray-0.1+git3356.gc9f02d85+~db20211214.g0a0added/tools/bitread.cc: In function ‘int main(int, char**)’:
/build/reproducible-path/prjxray-0.1+git3356.gc9f02d85+~db20211214.g0a0added/tools/bitread.cc:354:27: error: ‘constexpr decltype (std::visit((declval<Args>)()...)) absl::debian10::visit(Args&& ...) [with Args = {BitReader, std::variant<prjxray::xilinx::Series7, prjxray::xilinx::UltraScale, prjxray::xilinx::UltraScalePlus, prjxray::xilinx::Spartan6>&}; decltype (std::visit((declval<Args>)()...)) = int]’ is deprecated [-Werror=deprecated-declarations]
  354 |         return absl::visit(BitReader(in_bytes), arch_container);
      |                ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /build/reproducible-path/prjxray-0.1+git3356.gc9f02d85+~db20211214.g0a0added/lib/include/prjxray/xilinx/architectures.h:13,
                 from /build/reproducible-path/prjxray-0.1+git3356.gc9f02d85+~db20211214.g0a0added/tools/bitread.cc:24:
/usr/include/absl/types/variant.h:98:72: note: declared here
   98 | [[deprecated]] constexpr decltype(std::visit(std::declval<Args>()...)) visit(
      |                                                                        ^~~~~

and a lot more warnings (that turn to errors) about deprecated APIs.