accel/tcg: Reduce 'exec/tb-context.h' inclusion

accel/tcg: Reduce 'exec/tb-context.h' inclusion

Only 2 headers require "exec/tb-context.h". Instead of having
all files including "exec/exec-all.h" also including it, directly
include it where it is required:
- accel/tcg/cpu-exec.c
- accel/tcg/translate-all.c

For plugins/plugin.h, we were implicitly relying on
  exec/exec-all.h -> exec/tb-context.h -> qemu/qht.h
which is now included directly.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210524170453.3791436-2-f4bug@amsat.org>
[rth: Fix plugins/plugin.h compilation]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>