meson.build: Fix header detection on FreeBSD

FreeBSD requires sys/types.h for sys/sysctl.h, add it as part of the
includes when checking for headers.
Instead of splitting out the check for sys/sysctl.h from the other
header checks, just add sys/types.h to all header checks.

v2 [Emil]
 - add inline comment
 - drop bash/sh hunk

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Niclas Zeising <zeising@daemonic.se>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>