Add missing include to X86MCTargetDesc.h (llvm#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

(cherry picked from commit 7abf440)

Author: hageboeck

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

@@ -13,6 +13,7 @@
 #ifndef LLVM_LIB_TARGET_X86_MCTARGETDESC_X86MCTARGETDESC_H
 #define LLVM_LIB_TARGET_X86_MCTARGETDESC_X86MCTARGETDESC_H
 
+#include <cstdint>
 #include <memory>
 #include <string>