hw/sh4: Add missing Kconfig dependency on SH7750 for the R2D board

r2d_init() calls sh7750_init() so depends on SH7750.Harmless
at the moment because nothing actually uses CONFIG_SH7750
(hw/sh4/meson.build always compiles sh7750.c and sh7750_regnames.c
unconditionally).

Fixes: 7ab58d4c ("sh4-softmmu.mak: express dependencies with Kconfig")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210222141514.2646278-3-f4bug@amsat.org>

hw/sh4/Kconfig

@@ -10,6 +10,7 @@ config R2D
     select PCI
     select SM501
     select SH4
+    select SH7750
 
 config SHIX
     bool