package/exfat-utils: fix compilation error with musl

exfat-utils build would not build on Linux unless __GLIBC__ was defined.
Patch added to use more relaxed __linux__ instead.

See http://autobuild.buildroot.net/results/c60/c60d0f9a93c90d41c3c86c78b0a0



Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>