plugins: Include missing 'qemu/bitmap.h' header

Since commit c006147122 ("plugins: create CPUPluginState and
migrate plugin_mask") "qemu/plugin.h" uses DECLARE_BITMAP(),
which is declared in "qemu/bitmap.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240418192525.97451-19-philmd@linaro.org>

include/qemu/plugin.h

@@ -13,6 +13,7 @@
 #include "qemu/queue.h"
 #include "qemu/option.h"
 #include "qemu/plugin-event.h"
+#include "qemu/bitmap.h"
 #include "exec/memopidx.h"
 #include "hw/core/cpu.h"