Add missing include to X86MCTargetDesc.h (#123320)

In gcc-15, explicit includes of `<cstdint>` are required when fixed-size
integers are used. In this file, this include only happened as a side
effect of including SmallVector.h

Although llvm compiles fine, the root-project would benefit from
explicitly including it here, so we can backport the patch.

Maybe interesting for @hahnjo and @vgvassilev

Author: hageboeck

llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h

@@ -14,6 +14,7 @@
 #define LLVM_LIB_TARGET_X86_MCTARGETDESC_X86MCTARGETDESC_H
 
 #include "llvm/ADT/SmallVector.h"
+#include <cstdint>
 #include <memory>
 #include <string>