meson: remove false check for key_serial_t

meson: remove false check for key_serial_t

key_serial_t is defined in keyutil.h, which wasn't included in the header list
in the test, so the test always failed. We were always compiling stuff with
!HAVE_KEY_SERIAL_T.

We could try to add keyutil.h to the test, but then we'd have to first check if
it is available, which just doesn't seem worth the trouble.

key_serial_t should always be defined as int32_t. Let's keep the uncoditional
define, since repeated compatible typedefs are not a problem, and it allows us
to compile even if the header file is missing. If there's ever a change in the
definition, we'll have to adjust the code for the different type anyway, and
our compiler will tell us.