crt: ignore clangs builtin __rdtsc

mingw-w64-crt/intrincs/rdtsc.c

diff --git a/mingw-w64-crt/intrincs/rdtsc.c b/mingw-w64-crt/intrincs/rdtsc.c
index 3a5e056..fd9cb75 100644
--- a/mingw-w64-crt/intrincs/rdtsc.c
+++ b/mingw-w64-crt/intrincs/rdtsc.c
@@ -4,6 +4,9 @@
  * No warranty is given; refer to the file DISCLAIMER.PD within this package.
  */
 
+#ifdef __clang__
+#define __IA32INTRIN_H
+#endif
 #include <intrin.h>
 
 unsigned __int64 __rdtsc(void)