core: Fix GCC 6 compiler warning regarding left shift of negative value

core: Fix GCC 6 compiler warning regarding left shift of negative value

In file included from pulse/timeval.c:32:0:
pulse/timeval.c: In function 'pa_timeval_add':
./pulsecore/macro.h:303:28: warning: left shift of negative value [-Wshift-negative-value]
              ? ~(~(type) 0 << (8*sizeof(type)-1))

reported by Ubuntu gcc-6

gcc-6 adds -Wshift-negative-value (enabled by -Wextra) which warns
about left shifting a negative value. Such shifts are undefined
because they depend on the representation of negative values.

also works with -Wshift-overflow=2

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>