Fix SIZE_MAX compiler warning when using stdint.h

This new symbol may be defined by the compiler. If it is, avoid a compiler
warning when USE_STDINT is defined.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/linux/kernel.h

@@ -16,7 +16,9 @@
 #define LLONG_MAX	((long long)(~0ULL>>1))
 #define LLONG_MIN	(-LLONG_MAX - 1)
 #define ULLONG_MAX	(~0ULL)
+#ifndef SIZE_MAX
 #define SIZE_MAX	(~(size_t)0)
+#endif
 
 #define U8_MAX		((u8)~0U)
 #define S8_MAX		((s8)(U8_MAX>>1))