Add `:standard` to the stub flags

[Leonidas-from-XIV]

While running the CI tests for a PR in Yojson, I realized that bin_prot doesn't build on opensuse-15.5-5.2_opam-2.1 with the following error:

# /usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: xen/blit_stubs.o: warning: relocation against `caml_leave_blocking_section' in read-only section `.text'
# /usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: xen/blit_stubs.o: relocation R_X86_64_PC32 against symbol `memcpy@@GLIBC_2.14' can not be used when making a shared object; recompile with -fPIC
# /usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: final link failed: bad value
# collect2: error: ld returned 1 exit status

This is most likely due to mirage-xen not existing thus flags being set to ()

Package mirage-xen was not found in the pkg-config search path.
Perhaps you should add the directory containing `mirage-xen.pc'
to the PKG_CONFIG_PATH environment variable
No package 'mirage-xen' found
File "xen/dune", line 8, characters 0-215:
 8 | (library
 9 |  (foreign_stubs
10 |   (language c)
11 |   (flags
12 |    (:include cflags.sexp)))
13 |  (name bin_prot_xen)
14 |  (public_name bin_prot.xen)
15 |  (optional)
16 |  (virtual_deps mirage-xen-ocaml)
17 |  (libraries)
18 |  (preprocess no_preprocessing))

This PR adds :standard flags to the build in any case.

[aalekseyev]

We'll remove this stuff instead as #29 suggests.