exfat: fix build with musl

Fixes:
http://autobuild.buildroot.net/results/e1d/e1db07f0ea1e70c62f3294016c1b3a094de71d12/



The endianness handling functions in platform.h are protected behind ifdef
__GLIBC__ which musl doesn't define even though it does provide the
endianness handling interface.  Work around it by ensuring __GLIBC__ is
defined.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>