Remove unsigned type from 'mask'

Remove unsigned type from 'mask'

    gcc-4.3.2 warns:

    defaults.c: In function 'getModMask':
    defaults.c:2586: warning: comparison of unsigned expression < 0 is always false

    The line in question is

        if (mask < 0)

    and 'mask' is the return of wXModifierFromKey() which is an 'int' and can
    indeed return a negative value (see src/xmodifier.c), so let 'mask' be
    an 'int' too.