meson.build: fix builds with --buildtype=plain
meson.build: fix builds with --buildtype=plain

When building with --buildtype=plain, the build fails with the following
error:

    cc1: error: ‘-Wformat-security’ ignored without ‘-Wformat’ [-Werror=format-security]

-Werror=format-security was recently added in 43def1b3a5a by #745

whot/libratbag,libratbag/libratbag,libratbag/libratbag,libratbag/libratbag,whot/libratbag,whot/libratbag