accel: Include missing 'qemu/accel.h' header in accel-internal.h

The "qemu/accel.h" header is implicitly pulled in. Include
it explicitly in order to avoid when refactoring unrelated
headers:

  accel/accel-internal.h:13:32: error: unknown type name 'AccelClass'
     13 | void accel_init_ops_interfaces(AccelClass *ac);
        |                                ^

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250417165430.58213-6-philmd@linaro.org>

accel/accel-internal.h

@@ -10,6 +10,8 @@
 #ifndef ACCEL_INTERNAL_H
 #define ACCEL_INTERNAL_H
 
+#include "qemu/accel.h"
+
 void accel_init_ops_interfaces(AccelClass *ac);
 
 #endif /* ACCEL_SYSTEM_H */