package/rpi-rgb-led-matrix: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since the addition
of the package in commit e821078031e14df69e5142f23e9cb7b5a1ea0e77:

In file included from framebuffer.cc:20:0:
framebuffer-internal.h:83:10: error: 'constexpr' does not name a type
   static constexpr int kBitPlanes = 11;
          ^
framebuffer-internal.h:83:10: note: C++11 'constexpr' only available with -std=c++11 or -std=gnu++11

Fixes:
 - http://autobuild.buildroot.org/results/c035da0d183f21343f19f7dee982e8e73ee781e4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/rpi-rgb-led-matrix/rpi-rgb-led-matrix.mk

@@ -12,7 +12,7 @@ RPI_RGB_LED_MATRIX_INSTALL_STAGING = YES
 
 RPI_RGB_LED_MATRIX_MAKE_OPTS = \
 	$(TARGET_CONFIGURE_OPTS) \
-	CXXFLAGS="$(TARGET_CXXFLAGS) -fPIC"
+	CXXFLAGS="$(TARGET_CXXFLAGS) -fPIC -std=c++11"
 
 define RPI_RGB_LED_MATRIX_BUILD_CMDS
 	$(MAKE) $(RPI_RGB_LED_MATRIX_MAKE_OPTS) -C $(@D)/lib all