As recent glibc versions (>= 2.27.9000-36.fc29) also define 'struct
statx' which is also defined in linux/stat.h, compilation fails with
error:

| In file included from ../dt-utils-2018.05.0/src/crypto/digest.c:24:
| [..]/usr/include/linux/stat.h:56:8: error: redefinition of 'struct statx_timestamp'
|  struct statx_timestamp {
|         ^~~~~~~~~~~~~~~
| In file included from [..]/usr/include/sys/stat.h:446,
|                  from ../dt-utils-2018.05.0/src/dt/common.h:15,
|                  from ../dt-utils-2018.05.0/src/crypto/digest.c:19:
| [..]/usr/include/bits/statx.h:25:8: note: originally defined here
|  struct statx_timestamp
|         ^~~~~~~~~~~~~~~
| In file included from ../dt-utils-2018.05.0/src/crypto/digest.c:24:
| [..]/usr/include/linux/stat.h:99:8: error: redefinition of 'struct statx'
|  struct statx {
|         ^~~~~
| In file included from [..]/usr/include/sys/stat.h:446,
|                  from ../dt-utils-2018.05.0/src/dt/common.h:15,
|                  from ../dt-utils-2018.05.0/src/crypto/digest.c:19:
| [..]/usr/include/bits/statx.h:36:8: note: originally defined here
|  struct statx
|         ^~~~~

The linux/stat.h originates from the code that was copied from barebox
but is not explicitly required to be linux/stat.h instead of sys/stat.h
and we do not actually use struct statx.

Thus it is safe to simply replace occurrences of linux/stat.h by
sys/stat.h to fix compilation.

Signed-off-by: Enrico Joerns <ejo@pengutronix.de>