[SystemZ] Add header guard macros to vecintrin.h (#129170)

Add header guard macros to clang/lib/Headers/vecintrin.h. Found while
compiling the latest numpy with clang 19 on s390x which ends up
including vecintrin.h twice. The gcc version of this file has header
guards so numpy compiles fine with gcc.

Signed-off-by: Jonathan Albrecht <[email protected]>
(cherry picked from commit ddaa5b3)

Author: jonathan-albrecht-ibm

clang/lib/Headers/vecintrin.h

@@ -7,6 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
+#ifndef _VECINTRIN_H
+#define _VECINTRIN_H
+
 #if defined(__s390x__) && defined(__VEC__)
 
 #define __ATTRS_ai __attribute__((__always_inline__))
@@ -12861,3 +12864,5 @@ vec_search_string_until_zero_cc(__vector unsigned int __a,
 #error "Use -fzvector to enable vector extensions"
 
 #endif
+
+#endif /* _VECINTRIN_H */