libext2fs: fix potential build failure with OMIT_COM_ERR

libext2fs: fix potential build failure with OMIT_COM_ERR

This fixes following build failure when OMIT_COM_ERR is defined:

 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_clear_generic_bitmap’:
 lib/ext2fs/gen_bitmap.c:437: error: invalid storage class for function ‘ext2fs_test_clear_generic_bitmap_range’
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_get_generic_bitmap_end’:
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_get_generic_bitmap_start’:
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_unmark_generic_bitmap’:
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_mark_generic_bitmap’:
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
 lib/ext2fs/gen_bitmap.c: In function ‘ext2fs_test_generic_bitmap’:
 lib/ext2fs/gen_bitmap.c:559: error: expected declaration or statement at end of input
make[2]: *** [gen_bitmap.o] Error 1
make[2]: Leaving directory e2fsprogs/lib/ext2fs'
make[1]: *** [all-libs-recursive] Error 1
make[1]: Leaving directory e2fsprogs'
make: *** [all] Error 2

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>