package/pcm-tools: fix build with GCC 4.8.2

Add "-std=c++11" to CXXFLAGS, forcing C++ 2011 standard (it's
experimental in GCC 4.8.2 but goot enough to build pcm-tools).

Fixes:

  http://autobuild.buildroot.net/results/cf3c79f0c94be8a184d532570bdb1893090316a3/



Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>