lib/uuid.c: change prototype of uuid_guid_get_str()

There's no reason to require an appropriately sized output parameter
for the string, that's error-prone should the table ever grow an
element with a longer string. We can just return the const char*
pointer directly.

Update the only caller accordingly, and get rid of pointless ifdeffery
in the header so that the compiler always sees a declaration and can
thus do type-checking, whether or not PARTITION_TYPE_GUID is enabled
or not.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>