package/edid-decode: fix build with gcc <= 5

package/edid-decode: fix build with gcc <= 5

Pass -std=c++11 to fix the following build failure with gcc <= 5:

/tmp/instance-0/output-1/host/bin/mips64el-linux-g++ -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os       -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -g -DSHA="not available" -o edid-decode edid-decode.cpp parse-base-block.cpp parse-cta-block.cpp parse-displayid-block.cpp parse-ls-ext-block.cpp parse-di-ext-block.cpp parse-vtb-ext-block.cpp -lm
edid-decode.cpp: In member function 'std::__cxx11::string edid_state::dtd_type()':
edid-decode.cpp:192:17: error: 'to_string' is not a member of 'std'
  unsigned len = std::to_string(preparse_total_dtds).length();
                 ^

Also add a dependency on gcc 4.7

Fixes:
 - http://autobuild.buildroot.org/results/71e4a50542c4551797f1d0404d27244612b427d7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>