x86/hvmloader: Don't override stddef.h

Since c/s 73b13705af ("firmware: provide a stand alone set of headers"),
we've had a proper stddef.h.  Actually use it.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

tools/firmware/hvmloader/util.h

@@ -30,9 +30,6 @@ enum {
 #define SEL_DATA32          0x0020
 #define SEL_CODE64          0x0028
 
-#undef NULL
-#define NULL ((void*)0)
-
 void __assert_failed(const char *assertion, const char *file, int line)
     __attribute__((noreturn));
 #define ASSERT(p) \