bootm: fix pointer warning with lzma

Avoid warning:
cmd_bootm.c: In function 'bootm_load_os':
cmd_bootm.c:394: warning: passing argument 2 of
	'lzmaBuffToBuffDecompress' from incompatible pointer type

For 32 bit systems, this change shouldn't make a difference to code size
since sizeof(size_t) and sizeof(unsigned int) are equal.  But it does fix
the warning.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>