dosfstools: fix compilation with musl libc

The Makefile of dosfstools passes -D_GNU_SOURCE to get access to
non-standard definitions such as 'loff_t'. But this flag gets dropped
by the CFLAGS override, so explicitly add it.

[Peter: reword commit message]
Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>