plugins: Drop unused headers from exec/plugin-gen.h

Two headers are not required for the rest of the
contents of plugin-gen.h.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/plugin-gen.h

@@ -12,8 +12,6 @@
 #ifndef QEMU_PLUGIN_GEN_H
 #define QEMU_PLUGIN_GEN_H
 
-#include "exec/cpu_ldst.h"
-#include "qemu/plugin.h"
 #include "tcg/tcg.h"
 
 struct DisasContextBase;