util: Make Solaris implemention of p_atomic_add work with gcc

gcc is very particular about where you place the (void) cast
The previous placement made it error out with:

In file included from disk_cache.c:40:0:
../../src/util/u_atomic.h:203:29: error: void value not ignored as it ought to be
 #define p_atomic_add(v, i) ((void)         \
                              ^
disk_cache.c:658:4: note: in expansion of macro ‘p_atomic_add’
    p_atomic_add(cache->size, size);
    ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit a56c3e3a)