journal/fsprg: set -Wno-pointer-arithm only for that file

Both gcc and clang issue a host of warnings about void pointers used in
arithmetic. The warning must be ignored in that file to avoid multiple
warnings.

Makefile.am used to set this for all libsystemd-journal-internal.a sources,
because there's no finer granularity for warnings. Let's just set it for
this one file.