build: add abseil-cpp dependency to Nix devenv

After 8635d24 commit, the abseil submodule was removed in favor of
using pre-built abseil distribution. Installation of abseil-cpp was
added to install-dependencies.sh and dbuild image, but no change was
made to the Nix development environment, which resulted in error
while executing ./configure.py (while in Nix devenv):

  Package absl_raw_hash_set was not found in the pkg-config search path.
  Perhaps you should add the directory containing `absl_raw_hash_set.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'absl_raw_hash_set' found

Fix the issue by adding "abseil-cpp" to buildInputs in default.nix.

default.nix

@@ -117,6 +117,7 @@ in derive ({
   ] ++ (devInputs { inherit pkgs llvm; });
 
   buildInputs = with pkgs; [
+    abseil-cpp
     antlr3
     boost
     c-ares