build: fix mingw build

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>