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.

src/extent-scan.c

@@ -24,11 +24,9 @@
 
 #include "system.h"
 #include "extent-scan.h"
+#include "fiemap.h"
 #include "xstrtol.h"
 
-#ifndef HAVE_FIEMAP
-# include "fiemap.h"
-#endif
 
 /* Work around Linux kernel issues on BTRFS and EXT4 before 2.6.39.
    FIXME: remove in 2013, or whenever we're pretty confident

src/fiemap.h

@@ -52,8 +52,12 @@ struct fiemap
   uint32_t fm_reserved;
 
   /* Array of mapped extents(out).
-     The actual size is given by fm_extent_count.  */
-  struct fiemap_extent fm_extents[1];
+     This is protected by the ifdef because it uses non standard
+     zero length arrays.  Note C99 has the equivalent flexible arrays,
+     but we don't use those for maximum portability to older systems.  */
+# ifdef __linux__
+  struct fiemap_extent fm_extents[0];
+# endif
 };
 
 /* The maximum offset can be mapped for a file.  */