scrypt: fix compiler warnings on ARM

* cipher/scrypt.c (_scryptBlockMix): Cast X to 'u32 *' through 'void *'.

Patch fixes 'cast increases required alignment' warnings seen on GCC:

scrypt.c: In function '_scryptBlockMix':
scrypt.c:145:22: warning: cast increases required alignment of target type [-Wcast-align]

_salsa20_core ((u32*)X, (u32*)X, 8);
               ^

scrypt.c:145:31: warning: cast increases required alignment of target type [-Wcast-align]

_salsa20_core ((u32*)X, (u32*)X, 8);
                        ^

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>