build: avoid a fiemap compile failure on some systems

build: avoid a fiemap compile failure on some systems

* src/fiemap.h (struct fiemap): Adjust the previous change
to the fiemap_extents array, which would also require changes
to the sizeof calculations in extent_scan_read().
Instead, only declare the fiemap_extents zero length array
on linux, which is the only platform that references this member.
This avoids a compilation failure on systems that don't support
this non standard construct.  We don't use the equivalent C99
flexible array construct so as to have maximum portability.
* src/extent-scan.c: Cleanup. Remove a redundant #ifndef.