add dune project name

because of this opam pin/install error:

#=== ERROR while compiling bigstringaf.0.5.0 ==================================#
# context     2.0.4 | openbsd/x86_64 | ocaml-system.4.07.1 | pinned(git+file:///home/madroach/src/bigstringaf#master#5fc35f1c)
# path        ~/opam/system/.opam-switch/build/bigstringaf.0.5.0
# command     ~/opam/system/bin/dune subst
# exit-code   1
# env-file    ~/opam/log/bigstringaf-31496-dc6e87.env
# output-file ~/opam/log/bigstringaf-31496-dc6e87.out
### output ###
# Error: The project name is not defined, please add a (name <name>) field to your dune-project file.

dune-project

@@ -1 +1,2 @@
 (lang dune 1.6)
+(name bigstringaf)