x86: UV: fix header struct usage

Impact: Fixes warning

Fix uv.h struct usage:

arch/x86/include/asm/uv/uv.h:16: warning: 'struct mm_struct' declared inside parameter list
arch/x86/include/asm/uv/uv.h:16: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>