boblight: forcibly build against libstdc++

The uClibc++ implementation of stringstream operator>> is broken which
fails boblightd at runtime when parsing the configuration.

Due to the failed parsing, std::list unique() is called with an empty
list which causes the program to trigger a segmentation fault, this
is most likely another uClibc++ implementation error.

git-svn-id: svn://svn.openwrt.org/openwrt/packages@35027 3c298f89-4303-0410-b956-a3cf2f4a3e73