fixed gcc flags for caml sublibrary

[pveber]

I had the following compilation error:

$dune build
         gcc src/caml/struct_stubs.o (exit 1)
(cd _build/default/src/caml && /usr/bin/gcc -O2 -fno-strict-aliasing -fwrapv -fPIC -D_FILE_OFFSET_BITS=64 -D_REENTRANT -O2 -fno-strict-aliasing -fwrapv -fPIC -Wall -pedantic -Wextra -Wunused -Werror -Wno-long-long -g -I /home/pveber/.opam/4.11.1/lib/ocaml -I /home/pveber/.opam/4.11.1/lib/base -I /home/pveber/.opam/4.11.1/lib/base/base_internalhash_types -I /home/pveber/.opam/4.11.1/lib/base/caml -I /home/pveber/.opam/4.11.1/lib/base/shadow_stdlib -I /home/pveber/.opam/4.11.1/lib/jane-street-headers -I /home/pveber/.opam/4.11.1/lib/ppx_compare/runtime-lib -I /home/pveber/.opam/4.11.1/lib/ppx_enumerate/runtime-lib -I /home/pveber/.opam/4.11.1/lib/ppx_hash/runtime-lib -I /home/pveber/.opam/4.11.1/lib/ppx_inline_test/config -I /home/pveber/.opam/4.11.1/lib/ppx_inline_test/runtime-lib -I /home/pveber/.opam/4.11.1/lib/ppx_sexp_conv/runtime-lib -I /home/pveber/.opam/4.11.1/lib/sexplib0 -I /home/pveber/.opam/4.11.1/lib/time_now -I ../raw -o struct_stubs.o -c struct_stubs.c)
In file included from struct_stubs.c:11:
../raw/hdf5_caml.h:2:10: fatal error: hdf5.h: Aucun fichier ou dossier de ce type
    2 | #include "hdf5.h"
      |          ^~~~~~~~

This patch adds the -I gcc flag that is generated in the raw directory.