build: drop unused <sys/syslimits.h> header

build: drop unused <sys/syslimits.h> header

<sys/syslimits.h> is not standardized, so portable programs should
not need to rely on it.  If there really is something that we need
where <sys/syslimits.h> provided the limit but <limits.h> did not,
then that would be a candidate for fixing in gnulib.  But this patch
did not turn up any compilation failures on Linux.

* src/internal.h (includes): Drop unused header.
* daemon/libvirtd.h (includes): Likewise.
* configure.ac (AC_CHECK_HEADERS): Likewise.
Based on a report by Matthias Bolte.