package/apcupsd: add missing Config.in comment about C++ dependency

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/apcupsd/Config.in

@@ -63,3 +63,7 @@ comment "usb support needs a toolchain w/ threads"
 	depends on !BR2_TOOLCHAIN_HAS_THREADS
 
 endif
+
+comment "apcupsd needs a toolchain w/ C++"
+	depends on BR2_USE_MMU
+	depends on !BR2_INSTALL_LIBSTDCPP