build: fix mingw build
    
    Ever since commit e44b0269, 64-bit mingw compilation fails with:
    
    ../../src/util/virprocess.c: In function 'virProcessGetPids':
    ../../src/util/virprocess.c:628:50: error: passing argument 4 of 'virStrToLong_i' from incompatible pointer type [-Werror=incompatible-pointer-types]
             if (virStrToLong_i(ent->d_name, NULL, 10, &tmp_pid) < 0)
                                                      ^
    In file included from ../../src/util/virprocess.c:59:0:
    ../../src/util/virstring.h:53:5: note: expected 'int *' but argument is of type 'pid_t * {aka long long int *}'
     int virStrToLong_i(char const *s,
         ^
    cc1: all warnings being treated as errors
    
    Although mingw won't be using this function, it does compile the
    file, and the fix is relatively simple.
    
    * src/util/virprocess.c (virProcessGetPids): Don't assume pid_t
    fits in int.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>