target/arm/cpu-features: Include missing 'hw/registerfields.h' header

target/arm/cpu-features.h uses the FIELD_EX32() macro
defined in "hw/registerfields.h". Include it in order
to avoid when refactoring unrelated headers:

  target/arm/cpu-features.h:44:12: error: call to undeclared function 'FIELD_EX32';
  ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
      return FIELD_EX32(id->id_isar0, ID_ISAR0, DIVIDE) != 0;
             ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240118200643.29037-6-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu-features.h

@@ -20,6 +20,8 @@
 #ifndef TARGET_ARM_FEATURES_H
 #define TARGET_ARM_FEATURES_H
 
+#include "hw/registerfields.h"
+
 /*
  * Naming convention for isar_feature functions:
  * Functions which test 32-bit ID registers should have _aa32_ in