Don't include mono-dtrace.h when generating offsets

offsets-tool can run before mono-dtrace.h is generated which leads to a compiler error about the file missing.

mono/utils/dtrace.h

@@ -10,7 +10,7 @@
 #ifndef __UTILS_DTRACE_H__
 #define __UTILS_DTRACE_H__
 
-#ifdef ENABLE_DTRACE
+#if defined(ENABLE_DTRACE) && !defined(MONO_GENERATING_OFFSETS)
 
 #include <mono/utils/mono-dtrace.h>