x86: fix xsave build error

fix this build failure with certain glibc versions:

In file included from /usr/include/bits/sigcontext.h:28,
                 from /usr/include/signal.h:333,
                 from Documentation/accounting/getdelays.c:24:
/home/mingo/tip/usr/include/asm/sigcontext.h:191: error: expected specifier-qualifier-list before ‘u64’
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/sigcontext.h

@@ -264,9 +264,9 @@ struct sigcontext {
 #endif /* !__i386__ */
 
 struct _xsave_hdr {
-	u64 xstate_bv;
-	u64 reserved1[2];
-	u64 reserved2[5];
+	__u64 xstate_bv;
+	__u64 reserved1[2];
+	__u64 reserved2[5];
 };
 
 /*