util/path: Fix type which is longer than 8 bit for MinGW

util/path: Fix type which is longer than 8 bit for MinGW

While dirent->d_type is 8 bit for most systems, it is 32 bit for MinGW.
Reducing it to 8 bit results in a compiler warning because the macro
is_dir_maybe compares that 8 bit value with 32 bit constants.

Using 'unsigned' instead of 'unsigned char' matches the declaration for
MinGW and does not harm the other systems.

MinGW-w64 is not affected: it does not declare d_type.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>