libmount: check for struct statx

libmount: check for struct statx

Let's ensure struct statx exists before we try to use it. Checking for
the existence of linux/stat.h is not sufficient. This is because
[uapi/]linux/stat.h has existed since Linux 3.7, however struct statx
was only introduced with Linux 4.11.

The problem arises if one happens ot be using kernel headers from within
the aforementioned range, such as Linux 4.9.

  CC       libmount/src/la-utils.lo
In file included from libmount/src/utils.c:31:
./include/fileutils.h:100:33: warning: declaration of 'struct statx' will not
be visible outside of this function [-Wvisibility]
                    unsigned int mask, struct statx *stx)
                                              ^
libmount/src/utils.c:117:16: error: variable has incomplete type 'struct statx'
                struct statx stx = { 0 };
                             ^
libmount/src/utils.c:117:10: note: forward declaration of 'struct statx'
                struct statx stx = { 0 };
                       ^
libmount/src/utils.c:125:5: error: use of undeclared identifier 'STATX_TYPE'
                                STATX_TYPE
                                ^
libmount/src/utils.c:126:8: error: use of undeclared identifier 'STATX_MODE'
                                        | STATX_MODE
                                          ^
libmount/src/utils.c:127:8: error: use of undeclared identifier 'STATX_INO'
                                        | STATX_INO,
                                          ^
1 warning and 4 errors generated.
make[4]: *** [Makefile:11269: libmount/src/la-utils.lo] Error 1

Checking for the presence of struct statx explicitly avoids this
problem.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>