cppdb: fix comment dependency

As noticed by Thomas De Schampheleir.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/cppdb/Config.in

@@ -12,4 +12,5 @@ config BR2_PACKAGE_CPPDB
 	  http://cppcms.com/sql/cppdb/
 
 comment "cppdb needs a toolchain w/ C++, threads"
+	depends on BR2_PACKAGE_MYSQL_CLIENT || BR2_PACKAGE_SQLITE
 	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS