tests/kcmp: use <linux/kcmp.h> if it is available

tests/kcmp: use <linux/kcmp.h> if it is available

I see it as a good idea to stick to system headers whenever possible
in order to also check for possible discrepancies between strace's
internal definitions and definitions present in the system headers.

* configure.ac (AC_CHECK_HEADERS): Add linux/kcmp.h.
* tests/kcmp.h [HAVE_LINUX_KCMP_H]: Include <linux/kcmp.h>,
do not define KCMP_FILE and KCMP_SYSVSEM.